Watching movies in the theater nowadays is so expensive so I rarely do it. But I did see the trailer of Anne Curtis's new movie 'No Other Woman' and one of her lines there caught my attention. What she said was something like this:
What will you do if the only man that you loved is unfortunately married? I am not going to give up. Rather, I'm putting up a god-damned fight!
(I hope I got that right.)
Anyway, here are my thoughts on it. (Remember that I haven't seen the movie so this is more of a general thing. It's not necessary to take it in the context of the movie.)
First, a question: when did you find out that the guy is married?
If you know from the start that the guy is married, then you should know enough to stop yourself from developing any special feelings for him. (Needless to say, you shouldn't play with fire.) Now, you may say 'but I can't help it if I do'. Two options then: (1) choose to distance yourself from the guy, or (2) stay, but limit yourself to being just friends with him.
If you only find out that the guy is married after you have already developed a relationship with him, then you have to wonder why that is. It seems impossible it didn't come up in conversations at least. So apparently, the guy hid this fact from you? And I doubt that means anything good. The relationship has to end.
The bottomline is: you always have to keep in mind that it is not just your self that you have to think about. You also have to consider the guy, his wife, and their family. Therefore, in this case, better to accept and let go, rather than put up a god-damned fight.