Gay marriage right or wrong:
Or do you mean that gay _sex_ is helping spread AIDS? In which
case gay marriage may help reduce the problem. 
What is 'gay sex'?
I think (and this is a wild guess) that the original argument the person was attempting to make was that UNPROTECTED ANAL sex can increase the liklihood of AIDS transmission. That might be true. But if it is, it really has nothing to do with the sexual orientation of the two people involved.