Children of single parents grow up seeing a very different example of romantic love than those who grew up in a “normal” household. Namely, none at all.
Growing up with loving parents can fill you with positive examples of how to handle relationships when you’re older. Even having two parents who hate each others’ guts can demonstrate what NOT to do. Either way, it’s a learning experience.
But children of a single parent are left to gain that experience on their own. It’s a lot like taking on a new job with no training: you learn the most in the field, but it’s nice to have a heads up of what to expect.
That’s not to say that children of single parents are helpless; we just have to learn some things on our own, and overall, we value different qualities in our partner.