Gay/bi girls

I have a quite standard situation in relationships. I'm wanting sex much more often than my girlfriend does. When I'm mentioning this kind of problem through a joke, she gets offended or says that it's not that bad. I'm not trying to initiate anything anymore, because I know she don't want it. What should I do? I don't want to make her feel bad about it, cause it's not her fault that she doesn't desire sex as I do. But I'm also feeling down when I don't get what I want and feeling that she don't want me.