Will married/engaged/steady sims fall in love with someone else on their own?

Good evening. I have played sims 2 for about 2 1/2 years and know it very well. I recently started playing sims 3. My question is will a sim that is married/engaged/steady initiate an affair that would cause the other party to be a romantic interest on their own with no intervention by me? They will flirt up a storm and hug amorously (which btw in sims 2 would cause them to fall in love if their relationship was above 70). This hug does not cause them to fall in love and they don't seem to go any further. All they seem to do is piss off their spouse but it never comes to anything autonomously. I know I can make them do it but that takes the suprise out of it. The other thing that really annoys me is that after the spouse sees the flirt they are mad but then go over and ask the cheater about their day (wtf). If I saw my spouse do that I'd have no desire to flirt or kiss him let alone talk to him for a few days at least.

What am I missing here? Do they have to be in the negative on relationship before they will fall for someone else? Also, they don't make an attempt to apologize. I have a couple that have had really no contact for several days because their relationship has dropped to the 1/2 way point in green so now I am wondering if they are attracted to each other. I hope and pray we get the attraction system back somehow. I hate not knowing if they really like each other or not. In sims 2 I jumble my attractions around so every couple has 3 bolts for each other. Anyhelp in understanding would be great. Thanks in advance.