"How can we pass laws concerning the "homosexual lifestyle", including marriage and fostering, when we are not,as a society, fully in possesion of the facts concerning the health risks associated with that "lifestyle"?

What do those presumed 'facts' have to do with treating people fairly? AIDS and other STDs affect the heterosexual population also! There are many, many gay individuals who are NOT promiscuous and who **already** live together, lacking only the legal protections and privileges that 'straight' couple take for granted.
You are not going to 'protect' anyone from anything by continuing to withhold those protections and privileges from them. The careless ones will continue to BE careless, just as careless heterosexuals are.
There is some good evidence that allowing marriage can even improve that sad 'data' by giving folks hope and a reason to settle down and become monogamous!

You STILL offer very little in the way of positive suggestions.