It's too simple to say "only traits that help survival persist." We have vestigial parts in our bodies, we occasionally bite our tongues, and succumb to genetic diseases.

All natural selection means is that a certain setup is generally good enough to reproduce. So if you remember that "B" and "T" in the initialism there don't necessarily mean same-sex relationships only, they would be able to pass on their genes just fine.

As for others, maybe they don't. Look at the straight people who keep having gay babies. :P