I cannot see how one could consider every act of fornication to be on equal footing in terms of seriousness. I agree with other posters who compare one night stands, prostitutes, just friends, engaged persons, etc. as being of different levels of sin.
A sin is still a sin, he’s not saying it’s not sinful. He’s saying that the fact that a couple is engaged lessens it’s seriousness in his opinion. He didn’t say it turns it into a venial sin. It’s still grave matter.
In my less enlightened days, I had a girlfriend in college (an evangelical, by chance) who led me into more sex and drunkenness than I had ever encountered - her rationale was “I’m saved, it doesn’t matter.” I approached my priest, who started by telling me
ubi caritas … where there is love, there is God. And he then admonished me not to remain in sin - to marry this girl if I truly loved her, or to think about what that meant for me to continue sleeping with her if I did not love her.
Being 18, I listened to only the first part of that - and it was the wrong the answer.
Sex before marriage is still a sin - any sexual activity outside of a valid marriage is a sin. This is God’s plan, and the Church’s teaching.
If your fianceé heard him correctly, perhaps the priest had the good intention of thinking “if we cannot resist sexual urges in this culture, better to release them with those we love”.
Wishful thinking and incorrect thinking indeed. It is better to make love to a fianceé than to shack-fu with a drunken stranger, but only because shack-fu disrespects yourself and your fianceé so much more. Both still offend God. If you are to shoot someone and aim for the arm rather than the chest, you have only assaulted the person rather than killing them - but God is still offended even when the person you held at gunpoint is grateful for your merciful shot.
Best to stay out of sin - remember Rev 21:8. There’s no exception for the faithful Christian.
And lastly, we can and should fight the sexual nature of our culture. I gave up sex for five years before I married. Five years. What was most amazing was how it changed the way I dated - the “hot girls” were annoyed or even offended because they had come to know themselves as sexually gratifying, and I was having none of that. My wife, on the other hand, loved me (and loves me) for who I am.
Some say it’s better to “take care of” the urges yourself rather than sleep with another - this is better only in that you disgrace only yourself, not your partner. Best thing is - seriously - to sniff vinegar or take a cold shower. The physical symptoms of desire go away, and if you don’t feed the mental and emotional cravings, they succumb to you as well.