The "When did you choose to be heterosexual" is a bit of a bad argument in my opinion. But a lot of the popular arguments are bad ones.

Heterosexuality, I hope we can still agree (because the rhetoric wants to move this out of acceptable speech) is the "default" position. Sex is for reproduction, and homosexual sex doesn't reproduce. If heterosexuality is not in-born, the species will not last long.

So the question of why you leave a default position is more revealing question than asking why you chose to stay in a default position.

If you see someone missing a leg, you ask: "What happened to your leg?" The person might say: "I lost it in a car accident." "I got gangrene and they had to amputate." "I was born without it." "They couldn't stop the cancer." "I have a fetish where I didn't feel my leg was a part of me, so I used a chainsaw."

Could be any of those answers.

Do you ask someone why they still have both legs? Of course not.