Nowadays it's more do you want to become poz? All bb is risky, and not every top is neg even though they say they are! Whether they cum in your ass or not, doesn't matter... feels better if they do!
It's the act of penetration that makes it risky. All cocks, big or small can make small tears in your ass and cocks can have small cuts/ tears on the cockhead or leak precum as they fuck, even if they pull out and cum that juice is in you.... for me the more juice the better! Nothing like feeling a man cum inside your body, the throb of his manhood as he squirts... you don't get that with condoms! And feeling his juice inside afterwards, lingering in your hole.. his DNA slowly absorbing into your rectum...
I always think its the sign of a good top and fucking if the guy can make you cum 'hands free' and the sign of a good btm if you can keep taking his pounding after you have cum!
Happy bb fucking, no other way will feel the same!