If not genome editing, then at the very least if an accurate DNA marker can be determined that indicates if a fetus will grow into a gay person, with the ease of obtaining abortions I would guess a significant percentage of those fetuses will be aborted.
It's one thing to have a live and let live attitude about gays, but it's another thing to willingly bring it into your family especially since people are having fewer children than they have in the past.
Most straight people with children would like to see grandchildren.
Also, another thing that is going to play into the gay population is the fact that in the past a lot of gay men married women and fathered children.
If being gay is genetic, then that is one more area we will see less gay people being born. Fewer men these days are marrying women out of social pressure and thus will not have children.
Sure...we can pretend all these gay men and gay couples are still going to somehow be fathers without wives, but you know it won't really happen that much.
So, if being gay is genetic that's 2 different things that are going to severely affect the number of gays and what percentage gays make up of the population.
1. gay fetuses will be more often aborted if being gay can be determined.
2. fewer gay men will marry women and have the chance to father children and pass on their gay genes.
Whitedevil is actually correct in his assumptions the coming years we will see changes in the gay overall population.
Of course, he could be wrong if being gay is far more environmental than some want to accept in which case the gay population will continue along the same path as far as their part of the overall population.