Darwin might say that the more a species practices homosexuality the more likely it is to become extinct. Therefore, homosexuality is a negative survival factor.
Of course, if I were a liberal, I could also say that human sexuality is just evolving since our technical prowess has obsoleted the biological requirement and we now control our own path. While not classical Darwin, it would not break the theorem. (See Transhumanism.)
Then again, there are enough angels and pin heads to keep us all busy. Ultimately, no logical or moral argument will dissuade a liberal from his self-appointed path of destruction.