I'll admit, the romance part of sims life has never really been a high priority. I tend to just ... go with whoever strikes me and it seems to work out.
That said, I did add a character as a love interest on purpose once, but I was playing a generational game so I brought him in as a child. Through some MCCC shenanigans, he was adopted by the main sim's Uncle's husband so that he didn't show up as her cousin when it came time for relationships.
I need to finish updating my mods and get back to that save, they're in the middle of looking for said uncle in Selvadorada.