“My theory is that these particular individuals are self-loathing and consequently self-destructive.”
You are absolutely correct; they are miserable because deep down inside they know that what they do is wrong and a sin, and no amount of rationalization can make that go away. Because they cannot absolve themselves of their sin, their response is to try to drag everyone down to their level, in the final belief that if everyone is as miserable as they, everything will be OK.
I don't think it's a "misery loves company" thing. I think they feel that if they can silence the voices (and thoughts) that disapprove of their so-called lifestyle; then they can finally be comfortable in their own skin.
Some gays actually are comfortable in their own skin. They are the ones with a live and let live attitude who just want to sodomize each other in peace and don't like to get political or demonstrative about it. The self-loathing narcissists are the problem.