I agree with you. This is a far superior secular argument against legal recognition for same-sex marriage:
However, there is one good argument in the OP -- the one about the slippery slope from legal gay marriage to legal polygamy. The vast majority of gay marriage supporters turn into polygamy supporters as soon they start reading a few articles on the joys of polyamory.
Once established, nothing is more disasterous for women than letting men have more than one wife (hardly any women wind up with more than one husband). In countries where polygamy is at all widespread, married women see any single woman as a threat. Oh, and guys, they spend all your money so you won't be able to afford a second wife.
I don't think the author meant to praise the institution's record of "breeding" magnitudes of children, but rather the quality thereof. It astounds me that so many equal-rights defenders don't think about the importance of encouraging young men and women to get married and start a family. Civilizations take great pains to evolve rituals and rules around this.