The marital debt is how spouses should ideally respond to each other, not something one partner is supposed to call in as a coercion tactic and certainly not something to be taken by force.
The marriage debt is an older term the Church has done away with. And it did involve morally coercing a partner into sex. Moreover, I don’t think there’s any concept of it that shows complete generosity and love. We are called to responsible parenthood. By Humanae Vitae, large families should only spring out of prudent generocity. It acknowledges that with our awareness of how fertility works, we
are sometimes called to abstinence. Indeed, sex can be sinful in marriage if it is gravely imprudent.
I said this to my Dad ages ago when I learned about his vasectomy to save my mom’s life. She’s had four high risk pregnancies in 3 years. They sucked at NFP because my mom considered it a sin to refuse him. Generally they just said that NFP didn’t work. Had she not gone through the high risk pregnancies, they would have just said “We’re called to have a large family.” That was their mental work around. But her nearly dying challenged them into believing they had a moral exception to the rule.
I lacked a bit of sympathy because I was young, but I pretty much just yelled at him “If NFP doesn’t work well enough, then total abstinence. Sex isn’t loving if its going to kill your spouse. You are a murderer in that case. You shouldn’t let your lust control you to that level.”
Now that I’m older and appreciate that people have higher libidos than me and that my difficulties with chastity are more on the anorexic end of the moral spectrum, I recognize that he did his best. It would have been far worse had he chosen to ignore the health of my mother entirely. It just wasn’t a morally perfect step. He wasn’t ready to make it yet.
We ultimately have to understand where we all are on our journey and recognize how lust effects each and every one of us. For me, I have trouble with engaging in sex at all. I’d love to be more generous to my husband, but the ways in which I’ve tried to control it have only made things worse. So I know that if I am to grow in chastity I must be willing to be humble enough to express my love as fully as I am able to in the moment, gradually learning self mastery rather than letting my pride eat me away because I could have avoided XY and Z sin easily…just not in a way that wouldn’t have harmed our overall ability to engage in sex at all.
And from that experience, I know that telling anyone, even an elderly couple, that they should lessen their expression of love if it can’t be expressed perfectly is not what Catholicism is trying to preach at all.