I don't see why you can't ask for him to not have sex with her. If its just sex then he should be able to give that up for your comfort. You can always ask for what you would like to see happen, that is where negotiation begins. Then he can say what he would like to see happen and so on. It usually turns into a discussion about far more than the actual topic at hand and can be very valuable that way. All these things need talking about, so start talking and communicating. To me it should never be a question. I feel something, I want something, I need something... all equal communicating to my loves. It means I talk a lot!