That's bullshit. Single parenthood has been the leading connecting factor relating to SO many issues that it's undeniable unless your denial is a coping mechanism to avoid coming to terms with your personal failures and how said failures might impact others.

This ALSO applies to broken families, and children with step parents, though it's definitely at much lower rates. But it's clear that a lack of a proper two parent dynamic leads to serious issues, and while other people can fill that role it's not a guarantee. Equally so though, it's not a guarantee that both roles being filled means you'll turn out fine. There are other elements, but that doesn't mean that a leading factor shouldn't be identified.