BJJ:
You are very correct.
My first post was intended to answer why the bias exists. However, it is certainly true that the bias is unfair, and not even justified.
There are too many factors which go into people’s development to focus on just one and declare it dispositive. Children from whole families often have very difficult issues.
Additionally, nobody knows what happens inside the house. Imagine a couple who have a really bad marriage. If they divorce, their children will be “tainted”. If they stay married, their children will not be tainted- but will likely be exposed to far worse.
Maybe it is better to marry someone who wears the background on their sleeve, instead of finding buried issues later. I think it is probably easier to deal with what you know.