I think that's what the pope means. That and legal gay marriage will turn all straight women into carpet munching golf pros. Otherwise I don't see how allowing same-sex couples to marry—the civil right, not the religious rite—presents a threat to the "the future of humanity itself." The entire world population is not going to suddenly go gay and forget which hole shits babies if gays and lesbians and bisexuals are allowed to legally marry the same-sex partners we already have.
And this threat to the future of humanity stuff? Previous popes said the exact same thing about Lutheranism, democracy, equal rights for women, birth control, divorce, masturbation, abortion, etc., etc., etc. Just, you know, for the record and shit.