The Left argues that societies that allow Gay marriage have reduced incidence of HIV, as a way of advocating legalising Gay marriage.
Let’s put that in perspective with an analogy. Let’s say the Left is a skydiving school:
Parachute A has a high likelihood of not opening, Parachute B has a lower chance of not opening, and thus a reduced likelihood of you smashing into the ground. Of course they do not acknowledge the third option:
TO NOT JUMP OUT OF A PLANE