Neg vers bttm here and dont hesitate to take poz cum in me. I have a few rationalizations for what I do. First, "neg" really means "don't know", especially in a bathhouse or fuckparty pigsex atmosphere. Second, most poz guys are taking care of themselves, know their VL, and most are undetectable. It is probably lower risk to take a known poz load from a guy than it is to take it from a guy who is supposedly "neg". How else do you explain the younger pozboys out there? My bet is that they got pozzed by "neg" or unknown status younger dudes who dont have regular access to healthcare or a good city std clinic.
But those are just rationalizations. In the final analysis, when I am on all 4s getting a deep rough fucking, back arched like a slut, nose in my poppers and otherwise altered, I am pretty much begging to get knocked up, and the tops fucking love it.