I regret not saying out loud to a White Gay <tm> who was arguing that it was okay and in fact beneficial to trans people for cis people to play trans roles. I did call him out, but of course I thought of the perfect answer years later.

"When trans people can routinely play cis characters then we can have cis actors play trans characters."

I also thought later of "And how many straight characters has Ian McKlellan played?"

Like I said, I did call him out HARD and got a heartfelt thank you from a trans person (This was during a panel on LGBT representation and I was so mad) afterwards. But as always, I thought of better responses later.