Ah, quads. They are complicated. I have been in two myself.
I am wondering, are you genuinely sexually attracted to your quad boyfriend? If not, that could explain why you don't want sex with him. It's quite possible to have an emotional attraction to someone, even love them, without wanting to sleep with them. And I know that in a quad, it can be tempting to just go along with things in order to not rock the boat and make things more difficult for the other folks.
It is perfectly acceptable to leave the quad and just support your husband in continuing with the other woman. It's also perfectly acceptable to find your own boyfriend, or girlfriend too, your husband does not have to be a part of it. (This may seem like a no-brainer, but some people who are new to this haven't considered all their options.)
You absolutely need to tell at least your husband about your pregnancy and get the prenatal care you need. If it is possible that your boyfriend might be the father of your baby, he needs to be told soon as well. You do not have to continue in a romantic and sexual relationship with him even if he is the father. And please don't worry about taking attention away from the other woman. If she's good people, she will be happy for you.