I know you said it "could make sense to prefer straight couples", but beware of implicit associations when thinking that something like this makes sense. We're much more exposed to examples of heterosexual couples raising children than homosexual couples, and, as Harvard's Implicit Association Test demonstrates, this could easily lead us to thing that one "makes more sense" than the other. Increasing positive exposure to the opposite end of things can change our opinion. Harvard's IAT site has more information (check out esp. the Q's about the Black-White test): https://implicit.harvard.edu/implicit/demo/background/faqs.h...

Malcolm Gladwell also examines the phenomenon of implicit social adjustment in his book _Blink_ (http://www.amazon.com/Blink-Power-Thinking-Without/dp/031601...).