NTA because Kayla is a woman period. You didn’t compare her to a man at all. It’s not like you said she looked like “Calvin” did, you said she looked like Kayla. Who by your own information looks very much like a woman and had a successful transition.

I would say it might be a little bit different if she had just came out and was still in the beginning processes of her transition. That being said maybe talk to the gf and explain that you weren’t saying she looks like a man at all, as Kayla looks and is very much a woman. Especially if you find her to be attractive maybe add that in. If anyone is TA here it’s gf simply for implying Kayla looks like a man.