It's not as simple as survival of the fittest. Modern culture is way too sexually repressed and prude, so many, many people are incels who would be able to have sex in a healthy society. I think it's messed up because sex is a basic need, at least for males. Also, I don't have sex myself, but I don't have much of a libido these days so NBD. But it was a big deal for me when I was growing up.