I am probably the wrong person to answer this because I got all sorts of red flags by your use of the words 'forces me to lick his ass'. First off, no one should force you to do anything you don't want to do. If is truly what's happening, you need to get out of this relationship STAT.
Taking that out of the equation, he does have a pretty valid point. I, personally, will not allow a man to do things to me that I am not willing to reciprocate because this is only fair.
So...is it strictly the 'fishy' smell that turns you off? I suspect there is more to the equation because the smell issue can be fixed with soap and water. Your answer of girls not being as horny as guys is invalid because horniness has nothing to do with reciprocating a rim job. I think you need to figure out the real reason why you are unwilling to do this for your boyfriend. And it is okay if it just isn't your thing because not everyone is turned on by the same things. But in the spirit of fairness, you should stop him from licking your butt if you aren't willing to do the same. And if he's determined that you should do something that you don't want to do, you need to question if this is the relationship you are supposed to be in.