I'm fairly sure your sim wouldn't cheat on their established partner if they have the 'Family-Oriented' trait in CAS. I haven't ever had a Sim autonomously cheat on a partner, but then again, I usually force them to cheat. Do they have the 'Non-Committal' trait (can't remember if that's what its called)?
There is LittleMsSam's Simda dating app but I don't think that would really give you what you're looking for. That app is more of an online-dating -esque situation AKA the base games love interactions but with all the people in one area & a call away.
I know of a love-mod for children ('First Love') by LittleMsSam where crushing children get moodlets by being close to one another. I'd check that one out if I were you, however, it is just for kids.
