Sex and Anger (rant??)

To me, my husband asks for sex all the time. To him, we don't have sex enough. I get so tired of him asking that every time he asks I get more and more pissy about it and it makes me want it even less. I know this isn't common so I'm trying to find some good suggestions. Sex isn't fun/isn't something that I care about at all and I have no interest in making it fun or trying to find ways to make it work for me because I just have zero interest. We're totally on opposite ends of the spectrum. Basically I do it so it doesn't become something that will cause a divorce later in life. It's a bother to me, it seems like a job I have to do to keep him happy therefore ends up less interesting to me each time. I'm on my last day of my period, I just played roller derby yesterday and I'm sore and hurting (I have chronic back pain on top of all this) but he doesn't seem to have that in mind. I'm just so grumpy right now. To me it seems selfish of him but I also understand that men need it to function apparently so I'm trying not to be a complete bitch about it but omg I'm so tired of it and I hate knowing that this will be a part of life for ever. We've talked about our differences on the topic and while he seems to try to understand, the next time he wants it it's like all of what we talked about goes out the window or like he forgot my feelings on it. I don't feel like I'm right for him and that scares the shit out of me because we have two kids and want one more but this is a constant issue. Help me. I'm so distraught right now.