I would agree that, once gay marriage is widely accepted -- something I very much hope will happen -- it's a pretty short logical and ethical jump to get to polygamy. But in your lazy analysis, you're glossing over several factors that make it unlikely that polygamy is inevitable or even likely:
1) Whereas gay marriage simply requires a few deletions of gender references (as there is very little legal distinction between the "husband" and "wife" roles), polygamous marriage requires that every piece of legal code factor in the possibility of a third interested party.
2) Very few of the people fighting for gay marriage are taking up arms in defense of polygamy as well. In fact, many advocates are downright hostile to polygamy.
3) In turn, many of the most strident practitioners of polygamy (those who practice it out of a sense of religious duty) are hostile towards gay marriage, or any combination that involves more than one man.
In short, once gay marriage becomes a mainstream legal entity, polygamy will not simply be an unintended consequence. Despite being the next logical step, it would still face huge legal hurdles. You could just as easily say that, because most arguments for banning marijuana are equally applicable to cigarettes, we're just a razor's edge from legalizing the former or banning the latter.
Another problem with your simplistic, slippery-slope thinking: any such argument requires that you demonstrate that the bottom of the slope is a terrible place to wind up. If two men and three women did decide to enter into a lifelong, committed relationship, why is that horrible? It doesn't affect your relationship with Mrs. D'Souza. I don't see any widespread social ills arising. Hell, you can't even say that The Lord Almighty finds the practice offensive; there are just too many biblical counterexamples.
It's pure sophistry to pretend that the various restrictions on the practice of marriage are equally subject to revision. You list four:
1) "It requires that only two people be involved"
2) "It requires that they be adults"
3) "and not closely related"
4) "and (except in Massachusetts) it insists that one of the parties be male and one female."
Why not add some more?
5) "Marriage must be between members of species homo sapiens."
6) "Marriage must be entered into by the consent of both parties."
If gay marriage passes, are those two restrictions in imminent danger as well? Surely not. Allowing gay marriage won't open the floodgates for plural marriage, brother-sister marriages, man-on-child marriages, forced marriages, or marriage between a man and a box of pencils. Each is a different situation with different social and ethical ramifications, different supporters and detractors, and each would require separate changes to the law to enact. To pretend otherwise is just crap punditry.
I bid you adieu, and wish you nothing but poor luck in your defense of the Inviolable and God-Ordained Sanctity of the subset of possible marriage customs that were openly practiced and socially accepted in the 1950's-era U.S.
Though I failed to add it, I thought it was odd that homophobic views would be found 'neath a banner that says, "News Bloggers: Hard News, Raw Opinions, Penetrating Perspectives." Somebody over at AOL must have had trouble keeping a straight face when they pitched that slogan. Also, pay particular attention to comment #4 beneath the story; it contains some of the most insightful commentary on the sex-crazed gay menace that you'll ever see.