I don't think for a moment, that homosexual "marriage" will have any effect, for good or ill on HIV infection rates.
The MSM rates are rising in countries which have had Civil Union or homosexual "marriage" for years, uptake rates are low, and being "married" does not mean being monogamous.
Apparently homosexual "marriages" are often "open marriages" where partner exchange is encouraged. I believe that generally, this is a symptom of the fact that homosexuals are unable to produce their own children or an extended family structure.....which IMO is the reason that most heterosexual couples generally remain monogamous.

I see "GAY MARRIAGE" as a political construct, to bolster a bankrupt "liberalist" agenda, the whole idea being promoted by the news and entertainment media, to a confused and dumbed down audience of children.....of all ages.

The unfortunate advent of "gay marriage" bears no relation to the ever worsening infection rates in the MSM demographic, which seem to continue to rise regardless.