My sims usually have a romantic interest in high school or college (or both), or they'll pursue a romantic fling while getting to know someone who turns out not to be a good marriage partner (they hate kids or they're hot-headed or some other trait not conducive to "happily ever after"), and before too long they end up with "The One" from whom they never stray thereafter. Nobody in my sim marriages cheats (I am not the type to forgive cheating IRL so I don't allow it in my simworld), so once they're paired, they're paired.
Granted, some of my sims have more than a couple of flings pre-marriage, usually because they've decided to get the reward points from the aspiration for sleeping around—which is hilarious when they later get the Soulmate perk as well once they get married. I played around with the Club system once by having one of my male sims keep a harem called "Jonny's Honeys"...and felt a powerful need to shower afterwards.
