Duty Sex - What is the line?

I know this group does not recommend duty sex, and I don't want my husband to have any sex he doesn't actually want.... but now we seem to be in a murky area where it seems to me he is initiating sex he doesn't actually want, but he says he really does want it? What now? How do I figure out what is actually duty sex and what is not? I've told him so many times I need sex and he has turned me down so many times, I don't know how to believe him? It still feels like he doesn't want to be there, but he says he does? What now?