Hordak, I am very sorry that you are going through this. I did not read all the responses from the other forumers, but I did read your postings in this thread, and here are the impressions that I get of your wife:
1. She's lonely. I know this sounds crazy, considering that she goes clubbing with her friends and such, but I think that she feels lonely within her marriage and family life. Perhaps not physically lonely, but emotionally lonely. You mentioned that you two go out alone only once or twice a year, which doesn't sound enough to me. Can you find someone to watch the kids while you go out on your dates, once a week? I understand that your children are part of the family, but you don't need to take them with you everywhere.
2. She goes out so much because while out, she feels that she is experiencing the fun that she feels no longer exists at home. Also, she could be depressed.
3. Maybe she wasn't ready to handle the responsibility that comes with having a family, and now feels the need to make up for "lost times."
4. She's bored. It's not an excuse, but it could be the reason.
Maybe your wife doesn't know how to talk to you about these things, and when she does talk to you, she is not giving you the whole story because she feels held back somehow.