[lisper]

> why should one have to follow darwinian evolution

That depends on what you mean by "follow". You have to "follow" Darwinian evolution for the same reason you have to "follow", say, the law of gravity. That doesn't mean you can't build airplanes and spacecraft, but you still have to acknowledge and "follow" the law of gravity. You can't just glue feathers to your arms, jump off a cliff, and hope for the best. (Actually, rationalists aren't even gluing feathers to their arms. They are doing the equivalent of jumping off a cliff because they just don't believe gravity applies to them.)

[UPDATE]

> unnatural sex is wrong

The problem with that argument is that homosexuality is not unnatural. Many, many species have homosexual relations. Accounting for this is a little bit challenging, but the fact is undeniable.

https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals

[mensetmanusman]

Pointing to other species can be futile since they also eat their young and are horrible partners :)

[lisper]

Well, yeah, the whole "against nature" argument is bogus to begin with. But I think a counter-argument is stronger if it can be made even while accepting the other side's premises.