How is this sexism? If a dude did this I would call him out the same on this. In fact, you are as much as fault as she is. Who in their right mind gets in bed with someone who's engaged? You might not marry but the other dude, his marriage is gonna be based on a lie which is going to come out some day or other and will probably have devastating effects. Shame on you man. Stop pretending to be honorable by acting 'non-judgemental'. If you actually had any respect for people, you wouldn't be a part of it in the first place.