It's a terrible, thoughtless quote. Emphasis on thoughtless, because I don't think McCain has given even a moment of thought to the subtleties of talking about this. The second half of the quote is a non sequitur. Adoption is necessarily a context in which "both parents" are unavailable, so it makes no sense to cite the superiority of biological parents as a reason to prohibit adoption by gay individuals or same-sex couples.
"If you fall down and break your leg, there's no way out."