It's part of gay mythology. They believe that gays were "in the closet" for thousands of years, and so gay genes were passed down. Now that gays are "out of the closet" they are not having sex with the opposite gender, and so they believe, not reproducing.
The existence of "gay genes" is critical to gay mythology. If they weren't genetically gay, homosexuality would just be another behavior or choice. They can't emotionally accept that as true.
True. They cannot take responsibility for their behavior. One of their main arguments for total acceptance in society is that they supposedly were "born gay and can never change".
Of course they get help from the politically correct attitude of irresponsibility that has become pervasive in our society which we see in some areas of the scientific community, for example psychiatry and now "behavioral genetics". Which is total nonsense.