I would suggest there is no rational reason why,
Its more like, you like the taste of pussy, so you accept the risks, and bbbj are not that big of a deal, so your not willing to accept the risks.
When you desire something all logic and common sense go out the window.
Also when you break it down disease by disease and risk of catching something, I cannot remember specifics but bbbj, is more of a risky behavior then daty,
I remember one guy who said he would never do something like that, his tone suggested a phobia almost,
like I said rational thought tends to leave, when your in need,
Robin Williams said it best,
God gave man a brain and a penis, but not a heart big enough so both will work at the same time.