"three is a crowd" thought bubble

You don't have to have a mod. it is doable through regular gameplay. One sim has to live in a town that has the Free Love nap active. So like my sim was already married to her first husband so the most she could be was a romantic partner with the sim I wanted to be married to as well. So one day, her first husband gets a call from the boyfriend saying that he found a ring in her bag and he thinks she is going to propose and should he accept. So I made the first husband say yes you should and that made the wife and boyfriend immediately become engaged and they were able to have a real wedding too. I just had to make sure both husbands agreed to not get jealous by asking them to reconsider romantic boundaries. I didn't figure this out on my own. When I was trying to see if it could be done, I ran across a post somewhere in which someone mentioned the nap thing making it possible. It can take a while for that phone call to happen. You just have to be patient and observant of any phone calls that anyone, even children, in the family get. Someone will eventually get that call about the ring in the bag. The info seemed hard to find so hopefully this helps out others who might be doing large family things too.