This is a question that I posed to the missionaries yesterday, during a dinner thing at one of the members' houses. (I've actually been going to dinner with the missionaries quite a lot this week, it's been pretty good in helping me feel the spirit more).
Joseph Smith practiced polygamy and he got married to a 14 year old, Helen Mar Kimball. Why is this allowed? It's not something we'd want to associate with the leader of our church.
They said that polygamy was allowed, at that time, because God wanted them to produce more kids, so that the church would grow. If this is the case, why did he only have kids with his first wife, Emma Hale, and not with any of the other women he married?
They also said that there's a difference between getting married and being sealed, and that he was "sealed" to Helen but it wasn't an actual marriage. When explaining the difference between marriage and sealing, they said that even friends can be sealed to one another (the example they gave was if the two missionaries were to be sealed together, in a brotherly way - can two unrelated guys actually be sealed like this?) but that it isn't a marriage, and that Joseph Smith didn't have sex with Helen. I Googled this when I got home, and there is no evidence of them having sex. But, if that's the case, then why do so many people use his "marriage" to a fourteen year old as a reason why the church is allegedly corrupt?