That sounds like a strawman of liberalism. No one says any change is automatically for the best. Instead, liberals have principles such as the humanist one about the inherent worth of every individual. That's why all individuals have the right to express themselves in liberal societies.

If everyone became homosexual and we were still living in the wilderness, with no technological options, that would indeed extinguish our species. But technological empowerment is the result of liberalism via scientific freedom of thought, democratic governance, and capitalistic innovation. So the transhumanist scenario envisions our thoroughgoing power over nature, including our reproductive systems (via artificial insemination, for instance). Conservatives tend to demonize that scenario, but they're caught up in theological fantasies, according to secular liberals.