text: I'm bisexual myself, and have had serious relationships with both men and women.  I've explained to many people in the past the inherent difficulty that bisexuals may have in monogomy.  The closest metaphor I have come up with is this...If you had to live the rest of your life and only could have oral relations, and no penetrative relations, would you be able to make due with that?  Monogomy for a bisexual is like that...effectively cutting off half of their sexual interest.  It can be done, of course...but it's not their 'natural' way of being.\n\nI'm not saying this means that your friend is allowed to cheat on his girlfriend...honesty is the cornerstone of any relationship.  But you'd be surprised how many wives/girlfriends are aware of and accept a man who has the need to express their sexuality.  How he and his girlfriend make their choices is not really something that you need to expend a lot of energy trying to worry about.\n\nThe main concern I have is that you seem to imply that you and your friend are getting more serious than simply recreational activity.  If he is in a relationship and is still seeking another relationship on the side, this goes more to his character than his sexual orientation, and ending the whole thing is a very good idea.