A commenter here directed me toward a series by Michael Craven on the moral issues regarding homosexuality and same-sex marriage, asking what I thought of it. This post gives a pretty detailed answer to that request. I think Craven is better than most conservatives on this issue. He doesn't seem to have the screed that I often find in many of those who bother to spend much time on this issue. I don't think all his arguments are as effective as they could be, though, and a few seem to me to be real mistakes. Overall, I don't think he's actually achieved his goal, which is to provide an argument based on secular premises that establishes the traditional view of marriage in the Judeo-Christian tradition.
Craven's first part starts in the right place, by noticing the difference between what the Bible calls marriage and what most Americans call marriage. That's the most fundamental observation you need to make if you're going to have an intelligent conversation about this issue. I'm a bit disappointed in how he handles comments. Instead of pointing out that his commenters are tackling issues he hasn't gotten to, he asserts conclusions he hasn't argued for yet, and it makes it sound as if he's just making assertions that he can't back up.
Craven's design argument in the second part seems to me to rely on a mistake. He seems to think that evolutionary theory allows for a purpose in nature that affects morality. It's as if there's a purpose to procreate, and homosexuality prevents that. It's not as if homosexuality does prevent the continuance of one's genes. For one thing, gay people could become sperm or egg donors. For another, they could have a hand in raising their nephews and nieces, who may then go on to pass on genes that overlap with their own in a full enough way. So homosexuality isn't contrary to this supposed purpose of evolution anymore than singleness is. Even worse, it's a mistake to think evolution has purposes to begin with, at least if you restrict yourself to arguments that are secularly available without relying on theism.
I think it's kind of ironic that naturalistic evolutionary theorists can't resist talking in design terms, as if subconsciously they can't avoid attributing a designer behind the scenes, but they can't mean it literally and remain consistent. When Gould talks about selfish genes, he doesn't literally mean that genes have interests and that they consciously seek to promote them. So why should we think evolution has the purpose of procreation simply because it leads to a higher chance of procreation among those who survive to be able to pass on their genes? That could only be true if there's a designer (and it doesn't follow even if there is).
So he's trying to offer an argument that doesn't rely on controversial theistic premises, but I think this particular point fails in that regard, at least given that he doesn't spend the time motivating the thesis in a different way, such as arguing for a designer first on secular premises and then arguing that a designer who designed the world via evolution as contemporary biology holds must have intended procreation as a moral goal that requires some commitment to heterosexuality. That's at least not an easy task, and Craven hasn't really tried to fill out his argument in that way anyway. I happen to think the first step (a design argument) can be done. I don't think a natural law argument can succeed without that. But I'm also not sure a convincing natural law argument will work on this issue even given theism. The only versions I've seen lead to too much being immoral (e.g. voluntary celibacy or choosing to remain married to an infertile spouse) or involve a step to avoid such a result that seems hard to motivate independently (e.g. choosing to avoid a human purpose is wrong if you use the body parts associated with that purpose for non-natural goals but ok if you don't).
There's another gap in his argument in part 2 as well. If homosexuality is an unnatural perversion of something that has a designed purpose, it doesn't follow that it's morally wrong unless you again assume theism and our moral obligation to follow the intent of the designer as our purpose. The idea that we have natural purposes that we should follow goes back to Aristotle, so the argument finds good company in many who do not rely on theological premises. But I'm not sure they have a right to such attribution of purposes and to conclude moral properties as a result, not without divine intent as the basis of such a connection.