Why do they choose to marry within the Catholic Church if they are effectively barring themselves from the Sacraments and living in a state of unrepentant mortal sin?
Ask them that, and they look at you as if you're from another planet.
If it were up to me, I'd require that they live apart for six months or so before they married.
But, it's not up to me. My wife and I do this to try to share some of what's made our marriage as wonderful as it is with these couples. Most of them are in their mid-twenties, some have been married once before.
We try to talk them through why they ought to take intense sex out of the equation during engagement because the sex won't always be that intense and what will they do after the intense sex? In addition, another good reason to put the sex on the shelf is to make sure it's not the sex that's keeping them together.
We had one couple do that, and break up before the wedding date.
If I had slammed them with "mortal sin" and "hell", the guy would have gotten defensive and gone through with the wedding just to prove something.
It's a cultural thing. When I was in my early twenties, all the non-practicing couples I knew wanted to have a "church wedding."