(I can't seem to find where this was stated in the past -- so here goes! Apparently, I'm going to save some of you a lot of trouble in the future...)
Marriage involves a certain kind of physical intimacy. An intimacy that is only permitted within the bonds of marriage.
It is how married couples cooperate with God in the creation of children -- new souls destined for heaven.
While this activity ("sex") isn't the main activity within marriage, especially considered on a "percentage of your waking hours" basis, it is certainly not lacking or infrequent in a normal marriage.
We're not talking once or twice a year; we're talking once or twice a week, if not more often. And no, number of children or age won't change things *that* much.
And another point: Men will value the physical privileges of marriage more than women. In almost every marriage this is the case. There are various physical and psychological reasons for this, which I won't go into here.
Just for starters, being pregnant and/or breastfeeding (birth control pills do the trick, too!) will cause a woman to have far less desire for physical intimacy. It has to do with hormones. For the man, meanwhile, nothing is different.
How many women (non-Catholics mostly -- but not exclusively!) have assumed the man doesn't need the Marital Debt any more than they do -- and within a few months, he's got someone else on the side!