Welcome, I think you came to the right place to find some answers.
Honestly, this wouldn't be the way I define poly. He cheated on you several times, he pressures you to be OK with a purely sexual encounter (as he himself admitted that nothing will come out of this) while perfectly knowing that you are hurting and he isn't able to slow down, help you heal and rebuild the trust he misused and broke so many times? Sorry, definitely not poly in my book.
If he really wants to handles things openly from now on, he should stuff away all those other interests and wait for you to wrap your mind around this stuff. Slowly. Not saying: "Hey, I am poly, ah btw, there is this woman I want to lay, I told you I will be there for other reasons, but, as you now know that I am poly, that's what I really intended to do. That's alright, isn't it?" So wrong in so many ways ...
In your shoes I would demand of him to stop this mess and start concentrating on your relationship and your hurt. Because you have every right to feel hurt after this. He needs to do a lot of healing to make up for those times he breached your trust.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.