You do realize that it doesn't matter if you cum in the pussy or pull out and paint her tits, right? If you ride bareback and she's got crotch rot, you're going to catch it no matter where you drop your swimmers off.
That being said, your question is about as relevant as asking why squids ride in a tanktop, shorts, and flip flops. Why do most Harley riders insist on wearing a DOT approved do-rag instead of a full-face helmet? The answer is simple: because they choose to. It all boils down to risk mitigation. If you're popping wheelies on the freeway with nothing on your head but your goddamned sunglasses, I'm going to laugh at you. But if you're attentive and careful while riding without a helmet, chances are much better that you'll be ok.
It's the same with hobbying. You want to dip your wick uncovered in a $500 an hour
Upscale Escort, your chances of catching the drip are probably less than if you bareback every $60 hooktard on BP. If you choose to bareback her mouth and cover up for the main event, chances are you'll be safe. But eventually the law of averages is going to bite you and it's going to burn when you pee.
And ladies, remember that The Pill STOPS WORKING if you take antibiotics for, say, a case of chlamydia. So if you're riding bare and taking creampies, you might want to consider another form of contraception.
Condoms aren't 100% reliable...I've had one break in a session before. Scared the fuck out of me and into her.