It turns out there was a subtlety to what Zim was saying that I had missed:
Cooper says that same-sex couples pose no risk to society of accidental pregancy.
But that seems to presuppose that orientation doesn't change.
Let's start by assuming, as the Prop 8 people do, that sexual orientation is a choice, and can change over time.
That implies that anyone who's in a same-sex couple right now might later have sex with someone of the opposite sex. (I'm sticking to gender binaries here for simplicity.)
And that means that members of same-sex couples pose the same risk to society (the risk of accidental pregnancy out of wedlock) as what Cooper referred to as the “fertile member” of an opposite-sex couple in which one partner is infertile. (I apologize for following Cooper's lead, both here and in my previous posts on this topic, in oversimplifying and trivializing the difficult issues around infertility.)
Which leads inexorably to one of two conclusions:
Either same-sex couples should be encouraged to get married, just like opposite-sex couples with one infertile member;
Or our starting assumption was wrong, and homosexuality is not a choice, but an immutable characteristic.
Cooper's whole argument is to oppose same-sex marriage, so the first of the above two options is unlikely. Therefore, Cooper apparently believes that someone in a same-sex couple will never sleep with someone of the opposite sex, and thus that homosexuality is immutable.
I know that it's kind of pointless to try to figure out what the logical conclusions of Cooper's arguments are. The parts of his argument that are logical are irrelevant to the topic; the parts that are relevant are a web of non sequiturs. (And I'm guessing he would probably back away from the infertile-couples part of his argument if pressed.)
My impression is that he wasn't interested in building a logical argument; I think he just wanted to toss out any old idea that came to mind, in order to support his real argument, which was that gays aren't a suspect class and so only the flimsiest of excuses is needed for it to be Constitutional for the state to discriminate against them.
But I nonetheless think it's kind of fun and entertaining to look at what the logical consequences of some of the stuff he said would be, if taken seriously.