sims that are married or in a relationship

I think a lot of comes down to the story you make up in your head. As someone else mentioned, leaving some autonomy can help, cause then sims with a high romantic bar will seek each other out. I've had my sims autonomously serenade each other when inspired, give each other passionate kisses, or even duck out to a closet for a make out session! I also like to pay attention to my sims whims, and so that can help as well--seeing the little blurb above the sims head saying they want to kiss, whoohoo with, get married to, or make a baby with their significant other helps make it feel like the sim is actually invested in the relationship lol.

Aside from that, I'll just second what else has been said here--making little moments can go a long way.