man it's totally wrong. if you believe in the whole one-person-for-everyone shtick, then maybe yeah they owe it to themselves to give it a try. but how hard could it possibly be to just steer clear of your buds' ex-girlfriends?
it's so dag nab creepy to have one of your buds porking your ex-girlfriend, mature or not. i'm willing to wager that none of the people who say 'grow up, she can do what she wants' have had that sort of thing happen to them-- annoyance at this sort of thing is entirely irrational. (yes my ex-girlfriend is dating my ex-roommate, i don't talk to him anymore because they live in a different city and he annoys me now)
anyway, i wouldn't go trying to make any demands or compromises with him, because i guarantee you they feel completely justified. confronting him in some way may make it possible for you to be friends, it may make things awkward for him as well instead of just you.
if they're not serious maybe you can ride it out, but if they're serious and you're able to stay friends immediately and keep a pleasant living environment, then well i'm impressed.