It is said that children copy their adults' relationships. It actually feels quite logical, that the image you see of love (or not love) at home is the image you carry with you.
Before I continue writing, perhaps it is still important to point out: it is quite possible to break patterns. There are many who grow up and form healthy nice relationships - who in childhood saw the opposite. It often takes a lot of work to get there, though.
I think (and speculate freely as usual!) that it is the healthy thing we want to eat. Not a type of relationship. Because to a certain extent the image still lives on: twoness-married-all-life. That we have to show that particular image so that the children get a healthy way of looking at relationships. The facade is more important than what happens inside. Does it feel familiar? Even though we've seen so many miserable marriages, the image sticks so hard that it can still feel like a better option than a (maybe even loving) divorce. "For the children".
I know you know but: the children will not copy duplicity, marriages you manage to keep together, etc. They will copy or imitate (if they don't do the job of breaking patterns) the way you see yourself in a relationship, the value you place on yourself there and how much you compromise who you are. Because children of single parents can fix healthy relationships better than children of those who grew up with married adults, if they saw their single parent have nice, equal, healthy relationships with themselves and other adults (and the child with married parents saw a relationship where two people forget who they are just to continue the relationship).
It feels completely ridiculous to write the sentence above, but think how much value we (despite the fact that we are wise beings who understand this!) place in how it "should be" (look..) That many of us believe that it is the the image of relationships we give our children - not what it feels like.