Is it wrong to have sex when you're not in the mood?

Assume that you're in a long-term romantic relationship with another person. You will not always going to feel the desire to have sex. If your lover wants sex, is it wrong to do so? Might you have sex anyway, perhaps because you want to do something nice for your lover -- perhaps in the hope that your lover might do the same for you later? Many people seem uncomfortable with sex under those circumstances, i.e. absent a strong physical desire. Some claim that if you're truly in love, then your physical desires will fall into line. Hence, if you don't want to have sex, you might not really be in love -- or you might have other philosophical or psychological problems. Others think that to have sex even if not in the mood isn't right: it's degrading and might lead to resentment. Which of these views is right?