I have no problem admitting the scenario you present is very common, John. I was presenting the romantic ideal, but trust me, I consider myself a very cynical romantic at best. After 25 years in a troubled marriage, I suffer no delusions about sex or desire.
It is unrealistic to expect two different people to always have compatible or co-inciding sexual desires. But with good communication, which is the cornerstone of a good relationship, there can be a mutual negotiation so that both partners sexual needs are met to the best extent possible.
The type of negotiation I am suggesting though, is different than the one that many men and women use because the intent is different. For example, perhaps I'm not feeling any particular sexual desire at the moment just because of where my body is in its hormonal cycle. I have no problem with satisfying my partner sexually even though I may not be in the mood. That is where sex would be an expression of my love and affection for my partner. I care about their pleasure and satisfaction too. There is no element of keeping score and thinking, OK, now what are you going to do for me. There is no sense of begrudging duty or obligation. It is a gift.
If I didn't feel like it because I had a headache, then perhaps my mate would show his concern for me by rubbing my back or neck until my headache went away. That would go along way to getting me in the mood to reciprocate. But if it didn't, I certainly wouldn't expect my mate to think I "owe" them now. The intent and motivation is in caring for the happiness and well being of your partner. That's why I agree that great sex is the denoument of a good relationship whether it's a mutual quickie just for fun or an extended love-making session.
If partners cheapen sex down to the level of a bargaining chip, then they get the kind of sexual relationship that they are aiming for.