Despite (in my opinion) missing the point & also leaning a bit toward "yay heterosexuality!", what that poster said wasn't exactly insinuating anything bad about any other group.

When a child has a parent that's died, or a parent that leaves, or a parent that insert 90% of reasons for parental absence, it's natural that they'll have a harder time than anyone else. It's not that the widow would be a bad parent, it's that the kid would have to deal with the terrible situation of having a parent die.

If you were really wanting to get to the source of truth there, though, you'd also show how children that get adopted by a non-couple perform. If I had to guess, they're probably doing the best by far, because adoption as a single person is only really possible if you're incredibly stable.