The first thing that strikes me is that despite being in a relationship with not just one, but two people, you describe yourself as single.
There are non-sexual relationships, which typically involve asexual people. I assume a lack of compatibility orientation-wise would be similar to that. However, he needs to be comfortable with it, and it seems he might not be.
The situation seems pretty unfair (have sex with that guy, even though you're gay and therefore not comfortable with it, or lose the relationship with not just him, but her too), but there is no way I could justify forcing myself to have sex if I were you. I have no clue what you can do except make them see that it's not a fair thing to ask of you.