How did sexuality evolve?

Sexuality did not evolve. When humans were created (whether you believe that to be through God, evolution, big bang, or other higher power), they were created as heterosexuals that reproduce by way of intercourse with another human of opposite gender. While it is possible that traits have evolved, changed, or been formed / dropped, there has been nothing over the course of time to begin a change toward asexuality. The tails in humans, for example the tail has devolved to just a few fused bones now known as the coccyx as there is no use for a tail. There has been no change in our environment to trigger a switch to asexuality, which would require not just the change of an appendage but an entire re-wiring of the human system. The change would have to be so gradual that during the change, the human population would die out due to the inability to reproduce. . . . This information I gather from my education as a nursing student. This is looking at it scientifically speaking, which is what I interpreted was your angle from your question. You could also just say that God knows what he's doing and leave it at that.