Is there a way to make all my romantically involved sims be faithful except for a few that I designate as cheating when they want to? I have NRaas Story Progression, and I've set it to Flirt: Affair Adultery Chance -20 and Flirt: Affair Liason Chance -20, but my sims keep flirting with other sims that aren't their spouses (etc.) and asking each other on dates!

That would probably be handled by castes, but I'm coming fairly late to that party myself so I can't tell you exactly how to do it.

My guess is that you should set up a manual caste for cheaters and add your cheating sims to it (manually, since it's a manual caste)Then make another caste for partnered sims, which would probably work best as an automatic caste. Give the cheater caste priority over the partnered caste, and only allow the cheater caste to flirt with a sim who is not their partner.

@igazor could probably give better details on how to do all that, assuming I'm not completely off the mark here.

caste system should be helpful for some extend, as @Ghost wrote above. But if it is a matter of just one sim, particulary the one you more or less have under your plum-boot it's a rather overdose.

Anyway, going to http://nraas.wikispaces.com won't hurt.

@Ghost sdoj, and @ElaineNualla, I don't see how that would work, considering that in the caste rules they have the same rules that my sims are ignoring now. I'll try asking over at NRaaS like you suggested sometime. Thanks for your suggestions. :-)