Thanks for your thoughtful comments, poly.
I don’t think (3) is false, although my disagreement here may be merely semantic. Insofar that homosexual acts might be used prior to another act that could lead to procreation (as you describe), (3) is not violated, since the procreation is not located in the homosexual acts.
Sure, but then in what sense does gay sex thwart procreation? It’s conceivable, in a given society, that the people who have gay sex are the same ones who procreate (heterosexually, of course), and that nobody who refrains from sodomy has children. In such a case, how can one argue that homosexual acts necessarily thwart procreation? (We must have the “necessarily” there because the premise is not supposed to be based on likelihoods).
I think rather that (1) is false, because is gives a necessary but not sufficient component of the end of sexual acts. The other condition that philosophers tend to give is that sexual acts must be unitive. I think this argument, if properly formulated, would only require a few minor tweaks to a single premise to rule out a number of sexual acts as immoral. For instance, it should rule out acts that are “procreative” such as premarital sex or self-stimulation (while, say, watching porn) before ejaculating in one’s spouse. Neither of those acts are moral, but both are procreative. The problem lies in the fact that they are not unitive either.
This is all a good point, but Premise #1 could be easily reworked without mentioning unitive-ness, if we just put it in the form of a necessary condition, as you suggest. Or we could mention the unitive condition – which means we have to explain how gay relationships aren’t unitive in the relevant sense (not an easy sell, perhaps).
This is a tough argument to make. I believe it is sound, but (3) and (4) would require a lot of support in our social context. (By that I don’t mean that people would regard (3) as false. But (3), to have appreciable content, must be fleshed out with an account with what it means to be happy, so that (4) can be seen to have content as well. And that would require a lot of work.) So I do not think that this argument has as much of a shot at being persuasive as does the first you’ve given.
My thought isn’t that it would immediately persuade. Rather, I would hope it would plant a seed, so that then people would watch and see if homosexual activity tended to make people – even in ordinary terms – happy. There’s plenty of evidence that it doesn’t, although this evidence is being buried underneath cultural shouting right now. Once the shouting dies down and gay marriage is legal everywhere, the truth about the correlation could become more apparent.
Or else, if social acceptance of homosexuality severs the link between homosexual activity and unhappiness, I guess my argument would fall apart (or, as you say, require an obscure understanding of happiness). But in that case, I must say, I would be theologically puzzled. I don’t know of any other type of sin that doesn’t usually make people more unhappy and dissatisfied, in the long run. I don’t know why God would create an action with a tendency toward happiness, and then forbid it.
Either way, I think the flexibility and responsiveness to real-world observations is an advantage to my argument. It doesn’t insist on immediate conversion, but simply suggests what to watch for in the future.
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