One of my close relatives gave me this reason- humanity would not be able to carry on (ie. have children) if the government allow gays and lesbians to marry. And, to me, that reason smelled like bullshit- in Ancient Greece, if I could recall, in Athens or Sparta maybe? people were allowed to be gay, and did their society collapse? No, I think it thrived. There would ALWAYS be, to me, an equal amount of gays, lesbians, and heterosexuals. People won't and don't randomly flock to the gay/lesbian side after LGBT civil unions are allowed.
My point/rant is clear.