I was never very good at getting my Sims married off until University came along. Now I get them engaged in college and they marry as soon as they graduate. A lot of times I create mates for my young adult Sims in CAS. Sometimes they will marry another playable Sim if there is a suitable one around the same age. Once I had a college grad marry a Sim whom never went to college. Both were playables and had been sweethearts in their teens but he was an older teen and she was a younger teen so instead of making him wait until she went through college, I just let her grow up. I did let one pair of second cousins marry. Not too many of my Sims marry townies but strangely enough Benjamin Long married born-in-the games Sims in two different neighborhoods. I have never had a Sims marry an NCP.

This is my main concern atm because I started a new empty hood and don't plan on having townies, minus npcs because i'm playing with standard custom skins and maxis defaults except for supernaturals. Thankfully though someone made a huge sims ancestors thing which i'll be using to make gen 2 and up as the townies (with diff names) and then just using gen 1 to start the families and then add in downloaded sims as needed. My big fear though is inbreeding.

In the chemistry panel, the first Sim in the list is the one they have the most chemistry with. If you have ACR, you can check the exact numerical values. So for those of us inclined to following the chemistry path in these matters, the first Sim is the one Sim, I guess.

With a little help from ACR and the random nature of com-lot hopping gameplay and my own hopelessly romantic inclinations, my Sims choose for themselves. I don't get involved until the things seem set. That's when I'll marry them. Or break them up, depending what is needed.
My involvement in making and breaking the marriages follows one simple rule: who is it that they like the best? Three bolters are impossible to resist, and generally I also follow the chemistry path (3 bolts FTW! Or soulmates, or whatever.) Unless something stands in my way...
Who they like best can change in time and if it does change, I'll intervene. But if the Sim in question is a family Sim with children and a loving marriage, s/he will be totally faithful to her/his spouse at least until the kids grow up. I just think that this is what family aspiration means. So they will be set as faithful through ACR, no matter if the family is primary or secondary aspiration. Unless of course it's secondary to romance. I semi-randomize these things (aspirations) and atm I have a couple of rom/fam Sims. I never had those before and honestly I don't know what I'll do with them. I guess I'll have to wait and see. Romance Sims are notoriously difficult to marry, at least for me, and these Sims that also have family will be easier to pair off, I suppose. That secondary family aspiration they have will draw them into the bonds of marriage (again, what family aspiration means to me - they want to marry and have kids), but their romantic nature won't keep them faithful, since romance is primary aspiration here.

I could go on forever. But I won't. Writing this post I've realized that my "rules" are more complicated than even I thought. But here are the main ingredienets: aspiration, chemistry and circumstance. At times, they're joined with my current mood. But I guess this also counts as circumstance. In any case, it's never boring, never static, and I love it.

With multiple triple-bolters I use cyjon's pizza box, it allows you to see the number of chemistry. Anything over 90 is triple bolts, but you can see which one is a higher number.

Right now, its by who befriends who first, although there are other factors too.