how long have you been seeing him for? i share your opinions about sex, i think it is for 2 people who love each other. if he is able and willing to have sex with another girl then maybe you need to question how much he cares about you. your view of sex and his are obviously very different so i think this is something that is always going to drive a wedge between you two.

having said that, perhaps when he expressed his interest in threesomes he was talking theoretically. we all have fantasies, maybe it's an idea that turns him on but he wouldn't really carry it out.

you definitely need to talk to him about this and let him know exactly how you feel on the subject. Most men fantasize about this but it doesn't mean it's right to bring it up in a relationship.

Imagine him asking you to have a threesome with another man. The whole thing is not morally right, and never works out. It's about lust, and the American Dream ( I can have anything in my grasp attitude). In life we have choices, chose wisely it may make or break your relationship.

I don't think she wants to do it, at least thats what it looks like she is implying.

Fantasies in itself isn’t a bad thing, but sometimes guys can get carried away by them, and when the opportunity presents itself, they will take advantage of the situation or even create one.
He already tested the water by saying that he would like to have one, so I would strongly suggest that people in situations like this one should discuss what is acceptable and what isn’t in their relationship. If both partners don’t agree, then it is time to move on.
This post is almost 2 months old but I hope the original poster made the right decision. It’s very traumatic to find out your partner has been cheating.