dude.... here you go from experience....
you let your friend date her because you haven't hooked up with her and he makes a move.... instead of telling her how you feel about her also, you are a good friend to your boy and tell her he's a good guy.....
he sleeps with her.
then he dumps her.
then, since you still like her you go out with her.
you fall in love with her
you begin to hate your friend for what he did
you stop talking to your friend, that friendship is now officially over, you want to pretty much kill him (not literally but you know what I mean)
you stay with her but can never get past what has happened. it bothers you VERY MUCH.
that situation really happened to me. then when we finally broke up, my best friend (another guy all together) slept with her. we were not together anymore but I always will love her and he knows it. I can't believe he slept with my old lady. I would never have done something like that to him. I'll spare the details.