With a lot of different households, this does sometimes happen because not all relationships last forever. A lot of them do and there are plenty of stable, happy families and couples in my game who only have eyes for their One and Only. Generally if they get married, they plan to stay together through everything and usually that means they stay devoted to their spouse.
I would say that occasionally things get rocky or someone comes along who hits it off so obviously with a sim that things go differently. It's not something that I try to push them into at all but if it happens then I tend to accept that is the case and they figure out how to deal with it. There are also couples that just divorce and start over. That tends to happen with some of the pre-mades. At one point I played with Jeb Harris's family which was kind of a comedic household of ridiculousness. The dad was kind of a not great character and the mom stood up to him and kicked him out. Now, she is figuring out how to have a life of her own while raising her kids.
I would say that love triangles are a lot more common among my younger sims while they are figuring things out. There are a few households where the longtime married couples have drifted apart over time. If that happens, yes, they might end up with a love triangle. Like I said though, a lot of them are happily living their lives with someone they are completely committed to. There are just a lot of households with various outcomes rather than a goal of specifically breaking any couple up. I sometimes go with the flow of how sims interact with each other, so they can spontaneously fall in love when they aren't trying or they can stay focused on their special someone.