I think it is because the left is an unwitting tool of a communist conspiracy that was put in motion decades ago. When Khrushchev said "we will bury you", the apparatus was already in operation. I think all of the leftist causes that are harmful to the USA (homosexuality, blatant sexuality in media, the Sierra Club, Global Warming, etc) were originally started by the Commies as a way of eroding the USA. Just look at how how they destroyed a pro-American, pro-God org. like the Boy Scouts. Leftist idiots who support these causes think they are "doing good". They are just useful idiots of the commies.
The irony is that the commie regime that built the apparatus is long gone while their machine of destruction is still running. "We" are becoming "them".
The irony -- have the Communists, in any country, been kinder to gays? I believe homosexuality was a capital offense in the USSR.
Another irony -- gays & feminists seem to be the only ones left in those movements. Communism was able to get a leg up in Tsarist Russia because of wide masculine support. The disappearance of straight (usually white) men show how this movement is in a state of decadence.