At what point do you break up a couple?

I only do it for story, a simmer on here shared a idea and I liked it a lot I wanted to do it as well, because not all families are happy. I had two kids growing up and the idea was for them to marry and have kids but then the husband would cheat on a much more hotter sim, I wanted to do this and while on a date the boyfriend pushed his girlfriend autonomously from the mischief interactions. I want to play it along its slowly building up that he’s growing tired of her and will eventually cheat on her when their older. It adds for more fun