Why do sims cheat SO easily?

I don't know if this is just me, but I find that marriages, romantic relationships, and commitments don't really mean much to Sims. It kind of ruins the immersion a little bit for me, because I'd expect commited Sims in loving relationships to resist cheating perhaps? Is there any mod out there that fixes this issue or do I just have to force my Sims not to cheat every time?

FWIW: I don't have ACR or any romance-affecting mods. I've noticed this behaviour in non-romance sims as well.