Sure, it sounds like they're claiming that accepting gay marriage as legal will reduce a parent's chances for a biological lineage.
This one is pretty transparent.
For journalistic completeness, I'll link to Rick Santorum's op-ed in the Washington Times on the marriage issue in the election, here. But the "logic" in his writing doesn't follow through. The problems with children not having mothers and fathers comes from women becoming pregnant and men not marrying or staying married to them, not directly from gay marriage. The "consequences" are more subtle: it's more about young adults deciding that it's not important to have families at all, and then you have a sustainability issue. It's interesting because on the soap opera "Days of our Lives" were' about to see a situation where a gay man (Will) has impregnated a woman (Gaby) to prove his "masculinity" and may wind up raising a child with his male partner -- the partner (Sonny) happens to be the most levelheaded character in the show.