Maybe we try to explain evolution too much in terms of our own society. Many societies push everyone to get married and have kids, no matter what.

If there is a gay gene, it won't die out in any society where gay men marry women and have kids. The gay men might still have sex with men. If that helped to build relationships with other men, it might even help them and their children survive.

Want to throw one more twist into the theory? Here's one: Those gay men who married for convention and weren't really interested in procreative sex were probably a lot more likely to have offspring if they happened to be paired with highly androphilic wives.