My husband has the right to demand sex whenever he wants, because I have given him that right. Being a man, with the standard male ego, he does prefer when I am into it, though. Therefore, he doesn't demand that I do anything that I am physically unfit for. If I am ill and he really needs some relief, i.e., he will not be able to get to sleep unless he, uh, scratches that itch, we work something out. He does the same for me.
Also, he doesn't just walk up and say, “We WILL have sex now, whether you like it or not, byotch!” though he could and I might actually find it a turn on, LOL. What he does is start hitting on me in the various ways that he has developed over our relationship that he knows I like. If I truthfully tell him, “I will have sex with you but I am really not in the mood because of XYZ” He still does not say “Well, you WILL have sex with me for I am your Lord and Master!”, though again he could.
What he sometimes does in such an instance, though, is get very close up behind me, embrace me in a very dominating way and whisper in his most authoritative voice, something along the lines of, “You are the sexiest, hottest, most beautiful woman that I have ever laid eyes on. It is my greatest pleasure to have your love and submission. I want your body now and I will have it.” Of course, by that point, I have totally forgotten what XYZ were and it is a good thing he usually has such a tight hold on me because I have already melted right there in his arms...
So there is demanding and then there is demanding ...;)