"end of the line genetically"

Being gay doesn't mean the end of the line genetically as long as you do not impede (or even help in some way) the reproductive capacity of someone with some of your genes.

For example, a gay sibling who offered to babysit a straight sibling's children, or helped financially, so they decided to have children (or more children) (or even the gay sibling saved the life of the straight when they were drowning at 13 so they could have kids later or the gay sibling invented a new way to create cheap food so everyone in the general genetic area has more kids) would have some genetic fitness.

However-- generally people who like to have kids, and who hold social values that promote child bearing- will come to dominate any society that they are a part of within a very short time (100 years tops- but usually less than that). People who like to have kids and have social values that promote even more child bearing by their offspring would certainly use a no risk option to make their offspring straight.

That being said, there are some interesting animal studies showing higher rates of homosexuality when in overcrowded conditions. With our increasing population pressure, it might increase in commonality.