If evolution is sensitive enough to give us two kidneys via some indirect impact on the number of reproductive offspring we create, why can’t it drive a “software” change to give people the desire to have heterosexual sex?

The best answer I can think of is that sexuality is somehow very fragile for evolution to calibrate, so even natural selection isn’t powerful enough to select heterosexuality reliably.

But it’s hard to argue that persuasively with a biological basis which we don’t yet have.

Compared to the other evolutionary arguments for homosexuality it also doesn’t scale well to other non reproductive sexual behaviours.