Sim Loyalty & Faithfulness Incorporated into Nightlife?

[Ancient Sim]

This may or may not be relevant in terms of the upcoming Romance Mod update. 

I have two Sims in Uni who from the start had 3 lightning bolts for each other.  They were constantly seeking each other out (the usual Congratulations, Pillow Fights, etc.) and almost starving rather than leave each other's company.  However, not ONCE did either of them try to instigate anything romantic, although the male did attempt it a few times with a dormie (never came to anything as the dormie was always occupied at the time).  I then realised that the female still had a crush on a downtownie she'd had a blind date with ages before, who had fallen in love with her (she hadn't with him).

As soon as I removed this crush, she made a beeline for the other YA and that was that.  This is the second time I've played this dorm - the first time I removed the crush before they moved-in (forgot this time) and the two YA's were flirting from the start.  It seems obvious that loyalty to the former crush was what was holding her back this time, although it's interesting that one of my married adult Sims (3-bolts with her husband) had no qualms about falling for another married Sim she only had 1 bolt with.  The female YA is Knowledge and the male is Family; the female adult is Family and the male adult is Fortune, should this mean anything.   

I am wondering whether some new coding has been brought in that causes some Sims to be more loyal than others - it certainly looks that way.

[J. M. Pescado]

Nope, no loyalty or faithfulness. Sims fall in love with the first thing that flirts with them, regardless of whether they're already married. They are, however, inhibited from autonomously initiating romantic relationships if they already have one.

[Ancient Sim]

Well, Tiffany Beare (nee Sampson) and Jackson Ruben were both fairly-recently married with new babies, so presumably they're not particularly inhibited.  They were all over each other on a downtown lot - don't know which of them started it, probably Tiffany (Family Sim females are such tarts), but one of them certainly did.  Understandable in a way, because Kevin Beare is rather boring.  Then again, so is Tiffany.