scottyg

Don't evict your son... evict the girlfriend she is not family its not her home she has no right to be under your roof... and no right whatsoever to be in your son's room...  And if your son does not like that then he can learn to be an adult and get his own place were he can control who can spend the night....    Or to put in back at you... a mother that is cold heated enough to encourage/enable her son to spiritually self district and never become an adult does not deserve children

Are you likewise going to allow your son to live in your house (and hence endorsing of his activities) if he were to be making drugs in there?    How about running racist campaign?   Is he allowed to do just anything in your house he wants?
@JKing , I'm a mom myself.  I get how heart wrenching hard it is to put your foot down-- we love our children, and we don't want them to get hurt.  But lack of discipline is just spoiling and is going to cause only more damage in the longterm.  If you want to encourage your son to be an adult, then you need to have him move out to make his own dissections and deal with the consequences.  And yes, he's going to make bad decisions and get hurt and that's going to break your heart.  But that is what it is to be a parent.  

So, you admit he's not an adult.
Do a search on YouTube on Devouring Mother.  Enjoy.

Obviously not.  Otherwise, you won't have this conflict with your husband for 8 years.

We need to clarify a few things here:
- Age doesn't make a person an adult.  And because your son cannot yet support himself, he is still a child, not an adult.
-No where in here is anyone saying your son has to have X beliefs.   It's fine that he's not religious, and no one is trying to make him believe anything.
-You are endorsing his fornication: you're paying for the place for him to do it.  You're enabling it and defending it every time you take the stance you are.
- We are  speaking at to tolerable behaviors for a person utilizing a room in your house.  Any landlord has rules as to what behaviors/things are ok in the places they allow people to stay.  
What does a landlord do if a tenet does follow the rules?   Evict them.  

Kids have their own unique personalities, to be sure; but in general people also respond in more-or-less predictable ways to a given set of circumstances.
You have, over your husband’s objections, created a set of conditions over the last eight years that unfortunately have resulted in your son’s becoming something of a man-child—and an oversexed man-child, to boot.  And now, confronted with the reality of what you’ve created, you’re doubling down on the old dysfunctional attitudes and emotionally kicking your husband to the curb yet again.  
It takes a heck of a guy to stay with a woman who does that; and I hope you appreciate his meekness.  I don’t think I’d be so patient.  Men don’t appreciate being castrated in their own homes.

And use the word "fornicate" (in its various forms)!  That's the word he needs to hear.  He's not "making love".  He's not "having sex".  He's not "sleeping with".  He's fornicating.

"Sonny, we do not allow fornication in our home. You are welcome here as long as you abide the basic rules of our household, including the one about no fornication. If you insist on fornicating with Jane, you will need to move into your own place. That is not acceptable here."

Hallelujah! Finally, the young man can start taking responsibility for himself!
What your husband is attempting to do is called "parenting". It's the right call. What you are doing to your son, in all love and sincerity toward him as a caring mother, is called "enabling".

You asked for input, and you got it. Of course, you are free to view the situation however you choose. But it's worth reflecting on the unanimity of response.
Imagine if God said, "Well, you know, heaven is not just mine. It's for my children. It's the family home. Therefore, anyone can come in no matter how filthy he is or what he does." Would that still be heaven? Or would it be pretty much the world as it is now? The situation is not exactly the same, but it's similar.
Can your son smoke pot in your house? Can he shoot up heroin? Run a prostitution ring out of your bedroom? Cook meth? Can your son bring home young children to molest and butcher? Where does the "family home" concept end? At what point do you put your foot down and say, "No, this is pollution, and you can't bring that pollution into my home, even if you are my son"?
Assuming there is some point at which you finally balk and say, "No more!", how is that fundamentally different from telling your son, "No fornication here"? I suggest that it is not different, at least not in principle. We should live by righteous principles.

Age is just an arbitrary number used by the legal system to make parents legally accountable.  It doesn't mean much outside of that legal construct.
That tells you he is not yet an adult.  He can;t even support himself, how much more for a girlfriend?  And if she ever gets pregnant, that child will be a VICTIM of their immaturity and you are culpable because you allowed it in your home.
But the problem here is not your son.  The problem here is you and your husband's differing moral codes.  You need to straighten that out even if, at this point, it has almost become too late for your children to benefit from it, and may have already caused damage.  Hopefully, this doesn't blossom into something irreparable.
 
 

Your husband and yourself sit down with son and explain your position. If son does not agree, he is welcome to move out. If he says, he will move out, offer to help him find a new place. How about holding family prayers? 

I'm going to join the chorus on this one. From a practical standpoint - If he can't afford to live on his own, he can't afford to raise a child. If he can't afford to raise a child, no relations under my roof or anywhere else.

Yes and no. For me, this is a no-brainer. No fornication in my house. Period. No exceptions.

1. Your son is a dependent child.  He doesn't get to be an adult until he's paying his own way in life.  
2. Another things adults do, is have rules and enforce them...

Agree with @Jane_Doe and @LiterateParakeet 
Breaking the Law of Chasity is a huge deal. I would not allow it in my home. Your son’s eternal progression is important.

Your house, your rules.
If he wants to play the "I'm an adult now" card, then he can show you that by being an adult: start by getting his own place and paying his own bills.  
And that's completely ok.  
Warning I'm VERY going to be blunt here--
Your son is committing a GRAVE sin.  He is taking the scared powers God gave him -- the powers to create life itself and to bond a married couple -- and bastardizing it.  Throwing that sacred gift in the mud and treating it as if it was a nothing more than a child's play thing.  It is a HUGE disgrace to God, His gift, your son, and the girl involved.  And you, the parents, are endorsing it by allowing you to commit such acts under your roof with you paying the bills, etc.  
In my house, we respect God and His gifts.  You don't need to believe as I do (my husband is actually an Evangelical and I have atheists over all the time), but we're going to respect God.  I don't care what age you are: we're going to respect God in this house.  
You want your own rules?  Get your own house.

I am completely with your DH.  All piercings look trashy.  My wife went over 40 years without getting her ears pierced.  The day she did a bit inside of me died.
I have never seen pierced ears that enhanced the beauty of the person who has them.  She tells me that she feels obliged to wear them, because she is the only woman at work who doesn't...  errr at least didn't.  I thought that was way cool that she didn't, but now she does.  At least she takes them out immediately when she gets home and she never wears them on date night.
I made a rule for my daughters that they had to wait till they were 12 before getting their ears pierced.  My oldest daughter at age 11.5 asked if she could get them pierced early.  I said no.  She is now 21 years old and never got her ears pierced...  by her choice of course.  My second daughter got hers pierced at age 14 when my wife got hers pierced.  She usually does not wear her bobbits.  My youngest daughter is now 17 and has zero interest in getting her ears pierced.
I guess I like natural beauty over fake.  My wife puts on makeup before going to work or going out with her friends. She looks good, but she looks far better when she removes it and I can see her natural beauty.  Yes, there are a blemish or two, but I don't care.  She is gorgeous without.  And lucky for me, she doesn't put that stuff on on her days off or when we are on dates and she takes it off right after she gets home from work.
She has asked me several times how she looks in makeup.  The answer is you look great, but you look better without.  Why does she use it?  because it is expected by her friends and coworkers.
Honestly, I think most women look better without makeup.  So what if there are wrinkles and blemishes.  The natural face is generally the true face of beauty.

What difference does a belly ring make when the problem is a parent that doesn't seen to have any problem with their kids wearing crop and bikini tops?
That battle is well lost already.

It will, however, fall eventually; and so it’s best not to get too enamored of the idea of doing something merely because it’s popular.  

I guess it depends on the extent you wish to instill standards of modesty.
For the Strength of Youth - Dress and Appearance
But hey, since "it's the 21st century", you might as well allow your 15 yr old daughter to have not only a belly button ring, but also a nipple ring.  And a chain from the nipple ring, to a tattoo of the sexy cat thing on her stomach.  Nothing says "I wish to be respected for the quality of my soul and character" like that stuff.

You and your wife started this mess when you violated the law of chastity. In spite of this fact, the fact that you were permitted to have a Temple marriage is bottom line evidence of great mercy rather than abuse. Instead of counting your blessings, you turned the responsibility for the unpleasantness of your own created mess onto other people. Do you and your wife now acknowledge the fact that you and she are much more at fault for your problems than are the Church leaders? That’s the place to start.

I do not need to be in on the interviews. I know my Bishop and the kind of man he is. If I didn't, I would spend more time around him to get to know him better. No one can stop a parent from being in on the interview, but it should by no means be required. If anything, a second adult is needed for the Bishop's protection from false accusations. I have never had a Bishop use probing questions...and I have had at least 10 of them. There may be some bad eggs here and there, and some horror stories may be true, but I do not believe many of the stories out there...some are just too far out to be believable.

I do not need to be in on the interviews. I know my Bishop and the kind of man he is. If I didn't, I would spend more time around him to get to know him better. No one can stop a parent from being in on the interview, but it should by no means be required. If anything, a second adult is needed for the Bishop's protection from false accusations. I have never had a Bishop use probing questions...and I have had at least 10 of them. There may be some bad eggs here and there, and some horror stories may be true, but I do not believe many of the stories out there...some are just too far out to be believable.