What does it matter how its advance came about? Point is, we have developed methods and technology to allow people to become pregnant without having to have sex, thus your hypothetical is inherently flawed. IVF allows for the human race to never experience male/female sex ever again, and still...
If survival depended on light, I'd have whatever I could get.
So, if every one turned gay over night, our species would still be able to procreate, thus not die out after 100 years as you claim? (I would imagine if it was the only way our species could survive, we'd get pretty good at IVF, too...