Question 2 is an absurd red herring, and you know it. Allowing same-sex couples to marry in no way threatens humanity's survival, unless you take the insane next step that you imply, that same-sex marriage will be mandatory, and opposite-sex marriage banned.
We need and will benefit as a society when same-sex couple can wed, because marriage stabilizes couples, and stable households are good for society. This positive good has nothing to do with whether a marriage, Gay or straight, leads to children.
However, all marriages have the potential for children. Yes, same-sex couples cannot reproduce by marital coitus, but large numbers of hetero couples reproduce by assisted technology, adoption, and children from prior marriages. Get real.