I cannot believe that somebody as smart as you can't see the problem. If sex evolved, how could it evolve without some means of transmissible genetic variation?
Mutations. You're going to get way more variation over time with sexual transmission of variation than you would likely get from having to rely on mutations of self-replicating organizisms. Plus, a successful mutation will spread far more rapidly where there is sex than where there is none. If there is a successful mutation of one self-replicating "female", that mutation can be passed on only to that female's offspring. But if you have a successful mutation in a male, that success can be passed on to a much larger number of females.