Now to get a mult-celled organism to reproduce sexually you have to give the organism an incentive to fuse. That's a desire for sex. That's the pleasure aspect of sex. However, just a sexual desire may not get DNA reproduced because the organism can go hump trees and solve it's pleasure problem. That is sexual orientation.
Sexual orientation is an instinct.
This part of our brain is pre-verbal. All animals have sexuality programmed into them as an instinct. If human beings were the only species that did not that would be an anomoly. That would require two theories to explain how humans evolved from animals in all other respects except sexual orientation, which wouldn't pass Occam's razor. When birds, and then mammals evolved the X and Y-chromosomes appeared.