There are 2 ways in which I can see this being possible. However one has to realise that it is nothing to do with being gay. Homosexuality is just a correlate, not a cause.
One the article does mention which is the gay parents are more likely to be open minded. Open minded straight parents would of course be exactly the same in this regard it is just that gay parents are, on average, more likely to be open minded and accepting in today's culture.
Second is that gay couples can not get drunk and wake up surrounded by adoption papers. Every child of gay parents is thus a wanted child. Whereas straight couples have the danger of producing unwanted children. That said, as in the above case, straight couples who are responsible, infertile, or who adopt their kids would be in the same camp.