Do you do the same thing as me, and make them a household from the start?
Do you create your potential love interest and have them move into their own home? If so, how do you get them to eventually meet?
Do you let the game determine who your sim will fall in love with?
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These days, I'm usually playing saves with an ongoing critical mass of my own sims. If a historic pack comes out, that's going to be a different save.
I have a bunch of characters running around. I now generally let them find their significant others through meeting them naturally. I don't force characters to get together though there are a few times when I had a specific sim looking out for another sim "in the wild" to pounce on in the bowling alley or park or bar. They do have to meet each other out and about in some way.
At times, if I have a sim who just isn't finding anyone or who can't make a decision, I have them date a few prospects to usually make a choice. With, err, certain NAPs, cough cough, certain sims make fewer ... or is that more? ... choices. there are certain sims who have multiple ongoing love interests but most of them pick someone and make it work.
When I've made love interests for sims or edit townies into acceptability, I sometimes follow the unchosen wallflowers as they find their own lives. That has brought sims into my rotation.
At some point, I'm planning to possibly cull some of my households such as making them unplayable or possibly even permanently move away. I'll then be bringing new households that move in.
I've actually rarely made married couples to move into households in this version of the series. I've always had them pick someone in the game but that was usually someone I made or made over.