It sounds to me like this is something that gets him off. Don’t get me wrong, it’s astonishing that he doesn’t see why you’d be uncomfortable by this, but the act itself I think is something that some guys do even if they aren’t gay. I mean surely he has some porn on while they do this. 

Would I say he cheated on you? I don’t know, but you have every right to be grossed out and not be ok with what he’s doing. That being said it sounds like this is his kink, and if it’s not something you want him doing you should probably leave him, as this represents an incompatibility issue.

That being said I wouldn’t think it’s something to shame him over, and likewise I don’t think it’s something you should feel betrayed by.

But that’s just me. Honestly even if you tell him to stop it he probably won’t.