Threatening people with hell..."slamming" people with mortal sin. Reading the way you re-phrase the teachings of Catholicism makes it seem as though you believe this particular truth of Catholicism as something awful, outdated, out-moded, Medieval and cruel?
It's truth, right? And, judging from the possible consequences, it's a pretty essential truth, right? But somewhere between an essential truth and pre-marital counseling, it suddenly becomes something that would be mean, judgmental, insensitive and therefore it's inadvisable to let people know that they are toeing the line of Hell, lest they think that the Catholic Church isn't with it.
Have you ever tried explaining the risks and consequences of their actions, according to Catholicism? Or has your method worked 100 percent of the time from the very first time you ever performed this service, and so you've never had the need or curiousity to try educating these people on the faith that they're marrying within? Do you care, one way or the other, whether or not they know it's a mortal sin? Do you care, one way or the other, whether or not they stop this mortal sin? Do you think they are in danger of hell? Or do you think that, technically, yes it's the official teaching of the Catholic Church that they are committing mortal sin, but really, in God's eyes, it's not that big of a deal, and they just don't know any better, and you are sure that it has no affect on their salvation, and their actions are doing zero harm to the state of their souls and the measure of grace within them?
Why don't you counsel some engaged couples, and see if your approach allows you to have a fruitful discussion with them? See if it furthers the dialogue you've managed to build up over time.
I've explained to you what I do, but you don't like it. You want me to do what you would do.
No need to blow a gasket because we're not talking about eternal damnation to engaged couples.