I think the nature of the
anal sphincter is to remain closed shut by default. So, it may need some gentle coaxing to open up, loosen up a little? Anal massage
Then perhaps it wont hurt anymore if a man and a Tgirl does this
But nowadays anal sex is being tried by everyone. like:
Man + woman
man + man
man + Tgirl
Tgirl + Tgirl
The concept is pretty old too. More than a 1000 years.
Vatsayana's Kamasutra, Koka Shastra:
"Payu Maithuna" = Anal Congress