Second, the radical redefinition of marriage to encompass "alternative" lifestyles has led to fewer marriages in those countries in which it has been implemented. Dutch marriage rates, for example, have dropped steadily since gay marriage was legalized.
Third, the function of marriage is to provide a stable family structure in which to raise children. Gay couples are, by definition, incapable of reproduction without the intrusion of extraordinary practices. This is not to say that marriages that do not produce children should be dissolved (although one of the constant threads throughout most cultures is that a marriage may be dissolved or annulled due to the inability to produce children). Adoption, surrogates and turkey basters are not a substitute for fathers and mothers, and those who claim that they are lack an understanding of the critical roles that parents play in the raising of children.
Ultimately, this is not about marriage, but about elevating the immediate desires of an influential pressure group above the longterm stability of society. If you consider that harmless, then take a look at the European states where these "new ideas about social policy" have been implemented, with imploding birthrates, disintegrating culture and collapsing societies. Perhaps that does not seem very menacing to you, but it scares the hell out of me.