Boyfriend won't stop asking

Hi, I was wondering what your thoughts are on this situation. I've been with my boyfriend for over 2 years now and we get on really well most of the time but since we started trying anal sex after he told me he really wanted to give it a go he just won't stop asking me for it now.

I've told him I get no pleasure from it whatsoever and often it's very uncomfortable, but let him do it every now and again to keep him happy because I know that if I say no he gets really stroppy with me and I just don't have the energy for arguments and this is usually the only thing we tend to fall out over.

It's really getting me down, I make excuses so he doesn't keep on asking but I don't think I should have to, surely if I say I really don't enjoy it he should ask me a lot less/not ask at all anymore?

Do you think I'm right to be a bit upset about this or am I over-reacting?

Thanks guys, sorry for the long post.