The act does not CAUSE diseases. It can be a way of SPREADING a disease. There is a subtle difference. And the only way that happens is if one or both of the people involved has an STD or something of that nature already, meaning no matter where you stick it, you're likely to infect something.
Also to the people who say this goes against "natural law" you would think that natural law would have SOMETHING to do with what actually happens in nature because, guess what... Animals do it too! Link:
A Little Fellatio Goes a Long Way - ScienceNOW (I apologize about the title. Most of the other pages I was finding were littered with animals getting it on which I figured would get me in more trouble, so this was the best I could find).
The article mainly talks about it in Bonobos and fruit bats, but it's also been documented in goats, hyenas, other primates, and sheep.
However given that homosexual behavior as well as masturbation are even more widely documented in nature and still considered as going against "natural law" I have a feeling this won't make a difference either.