Looks like they don't need to worry about that anymore—thanks to science!
Using stem cell technology, reproductive scientists in Texas, led by Dr. Richard R. Berhringer at the M.D. Anderson Cancer Center, have produced male and female mice from two fathers.... The achievement of two-father offspring in a species of mammal could be a step toward preserving endangered species, improving livestock breeds, and advancing human assisted reproductive technology (ART). It also opens the provocative possibility of same-sex couples having their own genetic children, the researchers note.
Even if gay people had achieved our full civil equality before this breakthrough and everyone on earth decided to instantly go gay—because, you see, there's no such thing as a heterosexual, only gay people who have opposite-sex sex because they really, really, really wanted to join the Marines and then get legally married—gay people would still know that they needed to have a little opposite-sex sex every now and then just to make some more gay people.
But, hey, problem solved! Now everyone can go gay without having to worry about extinction. Thanks, science!