He keeps slipping out

So my boyfriend keeps slipping out while we’re having sex and then his dick will practically punch me in the the butthole. We can’t have sex after that because I’m not looking to get a UTI and even though he’s washed really good afterwards, I’ve gotten a UTI anyway.

I don’t know why this keeps happening and last night it happened again and he lost his cool. Cussing and then marching to clean up, he was pissed it happened and I don’t know how to fix it. When it does happen we’re usually in missionary .

Does anyone else have these issues? Did you fix it, how?