Consider what motivates someone to write "polyamory has much greater potential appeal, and poses a much deeper danger to the American family." I have no idea if "polyamory has much greater potential appeal" or, if it does, why it "poses a much deeper danger to the American family" than does polygamy.
Neither does Kurtz, for that matter. I am sure he has never knowingly met a polyamorous person, and hasn't done a minute's worth of research.
I'm polyamorous (or "poly") and have been for a long time. Trying to explain it to monogamous people is often either hilarious or frustrating, usually because while you're speaking, they are hearing words you're not saying. Or they confuse it with swinging, which is nine different kinds of funny. Suffice it to say, some people are not emotionally monogamous - that is, they can and do fall in love with more than one person and can sustain more than one romantic relationship with other like-minded partners. If you want more info, consult polyamory.org.
The basic mistake Kurtz makes is conflating relationships with marriage (or other legal recognition). My impression is that, outside of a few theorists, poly people are not focused on multiple marriage, for a number of reasons. First, they're usually just too busy trying to find or maintain whatever relationships they desire in a overwhelmingly mono world. Unlike homosexuals, for example, the poly haven't been discriminated against as a class, so there isn't a strong feeling that legal remedies or mainstreaming to gain acceptance are needed. The old-school polyamorous are hippies who rejected marriage; the younger poly crowd coming out of college are indifferent to it. Also, the reality is that, unlike gays and lesbians, poly people just don't have the political will or cohesion to lobby for a change in the laws even if they wanted to.
Kurtz could have found all this out in about half an hour online, but of course, he's not arguing in good faith anyway. He just wants another stick to beat homosexuals over the head with.