Malmesbury at Telescopic Turnip:
It’s pretty clear that, on a direct one-v-one cage match, an asexual organism would have much better fitness than a similarly-shaped sexual organism. And yet, all the macroscopic species, including ourselves, do it. What gives?
Here is the secret: yes, sex is indeed bad for reproduction. It does not improve an individual’s reproductive fitness. The reason it still took over the macroscopic world is that evolution does not simply select for reproductive fitness.
Instead, the evolution of sexual dimorphism is a long sequence of strange traps, ratchets and outer-world eldritch cosmic forces that made it somehow inevitable. So let’s talk about those things your parents never told you about.
More here.