Don't have a Sim-self, or a main character, because I tend to create a lot of Sims for each game I start, or one main family.
Most of my Sims I create, or most based on canon relationships from series I like, have been married only once.
However, one family I created was based on the series where the father has two wives at the same time. He is based on ancient times, where they had one legal wife, and usually a second wife. So, he is married to two women t once, and in canon has a son with each wife, for Sims 4, he now has a daughter with his second wife. His hands are full trying to balance both wives in the same household.
Some of my Sims are only married once, but I made them father/mother someone outside their union. Some examples is that I downloaded a Cloud, Tifa, and Aerith from Final Fantasy 7. Cloud is married to Tifa, and had two sons and daughter with her, but also fathers Aerith's two daughters as well. All made in CAS. I also made a Sim where the father is married to one sister, and had a child with her. However, he has a daughter with her sister. Not really a cheating scenario, because when I created them in CAS, the science babies weren't a thing, and I wanted her to have a biological child right away.
Except for a few CAS/experimental shenanigans, all my Sims are only married once.
