I think if you took a poll of married or single people with children and busy lives you would find that they don't have sex as much as they want and it has nothing to do with drive. When you don't live together and you don't have kids you probably have sex with the person every time you see them because when you see them that is where your energy is focused. If they don't want to have sex it is easy for them to say I can't hang out with you tonight. When you are married it isn't that easy. But my wife has never told me no. But I know the difference between when she is full of energy and ready to go or when she is just basically in it for me.
@Reesey said the same thing earlier. The other thing is if I know she has had a rough day and is tired I just wait until the morning or the middle of the night and wake her up. But after she gets a nap.