I have a few single & eligible Sims that I've created. I placed them in the world. So, any current households that I'm playing will be partnering up with the first Sims (that I've created) that they've met (by coincidence). I would say that, it is pretty much randomised although, it's within the same pool of Sims (one that I've created).
I always mentioned the importance of having an "attraction system" in the game. It's not only creating an interesting story/drama, but also, the chances to find the most compatible partner. I don't know when, but I hope that some day, it will come in a patch.