It doesn't have to end.
For the past eleven months, I've been involved with my best friend's husband. I'm single and was her maid of honor in their wedding two years ago. We'd had a little fling before they got married (while they were living together) and for a little while after, but we ended it. Then during the holidays last year, we rekindled it and got much, much more serious. It's not just s** but the s** is absolutely spectacular. He needs me more than her, and it's always me he comes to when he wants to talk or needs support or looks for help dealing with her. She's a disaster area most of the time and always sort of has been that way. Sometimes I have trouble dealing with the fact that he and I are together and have such a strong bond. They have two kids (one from before they got married and one just barely after), so I don't see him leaving her, at least not soon, but I honestly don't care. I can maintain this relationship, and keep him happy, forever. I don't need him to leave her to prove anything to me: what we have is better than what they have. And to be truthful, what he and I have is better than any marriage any of our friends have. It really doesn't have to end.