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
I'm sure you would be able to predict what a rationalise will say when you ask them what future they prefer: one where we maximises for the number of humans or one with fewer humans but better lives