I Don't Believe in Marriage
I (39F) have never been married or in a serious romantic relationship. That being said, I don't believe in marriage. One reason being my parents' own divorce during childhood, which was because of my dad having an affair with my stepmom (which still affects me and my siblings after 30+ years). Also, the fact that many marriages end in divorce for countless reasons.
To make matters worse, if you're in a religion, culture, etc., where divorce is forbidden or even more stigmatized, and your spouse turns out to be adulterous, abusive, or if the two of you drift apart and aren't compatible anymore, too bad you're stuck with him/her until one of you passes away. Had you been born in an earlier timeline when divorce was forbidden or difficult to get and highly frowned upon, same thing; you would've been stuck with the other person until death even if he/she cheated on you, was abusive, you drifted apart, or whatever else.
Of course people can fall in love and be together for a long time, but they don't have to "make it official by getting married."