It depends, first time is not very good, you are pushing inside of a hole that is only meant to be exit way.
It is not only about it hurting or not (not even about feeling like you can't golf back your shit, that is how you feel at first), when you learn how to relax and how to prepare your body it still needs some more time to really get usted to it. It's like the body knows something is not right and you start getting chills and feeling sick.
When you have been doing it carefully and you have been teaching your body that it is sex (by giving the body pleasure through other body parts at the same time) then it gets anjoyable.
I am a female, so I can compare both holes. The normal one is closer to the sexual receptors, but it has much lower sensibility. Through the back is harder to get that sexual pleasure, but you can feel it much better. There are women who prefer the back one.
as someone who is gay... most couples do switch. like real people, in a lot of yaoi there's so many assigned roles which just shows a bad presentation of real gay relationships. it's not always "uke" and "seme".