You might enjoy it,some girls do,alot of girls do do it to please their boyfriends or because there is less chance of pregancy which is partly true but unprotected anal sex still has a preganacy risk but a smaller one.Plus you have std' that are more risky if the bum sex is unprotected.
,it really is a mental thing and you need to be relaxed .Having a few drinks helps. However I was watching some sexual advice video once and the doctor was talking about anal sex.And she said some people have a kind of anal passage structure that makes the enjoyment of anal sex impossible ,difficult painful etc and it doesn't matter what you try.
Just try it, for the majority of girls providing he doesn't buy into how it happens in porn, ass bursters/ass destroyers etc etc it will either be enjoyable or bearable but not as pleasurable as vaginal sex . It is good to do things to please your partner in bed. For a minority it will not be, ever, and in which case he will have to accept that.
Personally if my girlfriend wanted to try something in bed providing it wasn't bi with a male, I would do it even if I wasn't that keen on it, I would try it once but if I didn't enjoy it or it was painful never again. Give and take in bed is best in my opinion. Relationships tend to fail because people just care about their own needs. That is why it's best to give something a go. And if he won't accept it if you don't like it, then he needs to get over it.