A few things could be going on here, and its a case of finding the issue.
Lets put this into perspective first, how many times in your relationship has he asked for sex and then been told no you have headache, not in mood etc? Then think about why you weren't in the mood, it could be similar.
I remember a time when constant rejection led me to not be interested, as I wasn't going to keep being told no, so I stopped asking. Then i thought, bugger this if you want it when you want it and how you want, i'm not planning on doing that just to help you, when it was no to my needs.
the way through it, is talking, the maybe a drink or two once its been talked over, and then maybe you can try again.