if people aren't getting enough sex in their relationships, it's usually a sign that something else is wrong, not the sex per se
This - and also the way both partners react to a difference/lull in sex drive is important. If it's accepted as a natural state of events when children are young, work is stressful, or for whatever other reason, and approached respectfully if it goes on for longer than this, it wouldn't be a problem. The problem comes when the partner who wants more doesn't respect the other partner's lack of desire and starts pestering/pressuring for sex, which starts off a vicious circle of resentment and rejection.
I suppose a different way of wording this is that in a situation where it becomes a problem, one partner (I'll use man just for simplicity, but it happens both ways) takes it personally that his wife doesn't feel like sex because he believes she should always find him attractive and want to have sex with him. So he sees it that she doesn't want him - where actually it's nothing to do with him, it's most likely because she is tired, stressed, has a lot going on or whatever, and the fact she's married makes no difference, if she was single or in a new relationship she probably wouldn't feel like sex either. (Although hormones help in a new relationship obviously, if she was that tired, stressed, etc, you wouldn't start a new relationship anyway. But I digress.) Really he should be seeing the other factors rather than "I don't get sex any more" and wanting to help relieve her stress, tiredness or whatever because he cares about her and not because he wants to have sex again.
Then of course some men have the belief that women don't really enjoy sex anyway so why should they have to be in the mood - but I don't think there's much point discussing them in this context, because (hopefully!) that is a rare and draconian view and the instances where one partner is just being a bit selfish and quick to take things personally rather than seeing it from their partner's POV is more likely.