Here's some applied ethics:
As many of you may have heard Bush's State of the Union Address, I think this is somewhat relevant. I remember attending two discussions by Robert Audi, one at my old undergrad and one here at Western. At both, Audi mentioned his principle of secular rationale. To give my best reinterpretation (I'm sure you can find it in a journal somewhere, perhaps I'll look it up), I will say that his principle basically says that:
We should only endorse a law for which there is a prima facie secular reason for doing so.
So, imagining the abortion debate, we cannot use the claim that a fetus has a soul (if "soul" is to be interpreted as a Christian or otherwise religious term). So, anti-abortionists cannot use this as a reason to outlaw abortion. Note, however, that anti-abortionists can feel this way more strongly than secular reasons, but there must be secular reasons that they put forward (hence the prima facie component). Regarding abortion, the secular reasons put forward are interesting and worth debate, but we should keep that to a different post.
My interest is to talk about homosexual marriage. Perhaps there are secular reasons for keeping homosexuals from having marriage rights, but here is an excerpt from Bush's State of the Union address:
Our second great responsibility to our children and grandchildren is to honor and to pass along the values that sustain a free society. So many of my generation, after a long journey, have come home to family and faith, and are determined to bring up responsible, moral children. Government is not the source of these values, but government should never undermine them.
Because marriage is a sacred institution and the foundation of society, it should not be re-defined by activist judges. For the good of families, children, and society, I support a constitutional amendment to protect the institution of marriage. (Applause.)
Although I think the quote speaks for itself, I think that logical analysis can make clear what Bush is saying.
First off, he seems to separate "many of his generation" from all of his generation (plus the rest of America). I believe it is Bush's duty to defend the rights of all of this nation's citizens.
Second, it seems that by supporting a ban on gay marriage, we are keeping the government from "undermining" our ability to bring up "moral children." What assumptions are we making? Is homosexuality immoral? According to whose standards? I can think of several in particular for our society: the most common versions of Christianity. Keep in mind Audi's principle. I have yet to find a (moderately reasonable) secular reason for thinking that homosexuality is immoral, although I would be interested to hear one. For obviously flawed secular reasons, see further comments.
Third, Bush says that marriage is a sacred institution. It is also, a legal and societal institution. We must not conflate these. Perhaps we should protect marriage as a sacred institution (we can give Church's the right to deny marriage to certain [i.e. homosexual] individuals), but we must also understand the motivations for marriage as a legal and/or societal institution.
As a legal instution, the purpose of marriage is to offer people protection and give them rights for various reasons. It is well known that when people get married, they promise to give up certain rights in the interests of the other (in economics, we can think of marriage as an opportunity cost). For example, a housewife may promise to stay at home and raise children. She is giving up various opportunities for the sake of another individual. That being said, if the other individual should choose to renege on the marriage, he has affected the other in a negative way, and is duly responsible for such negative affects. This can be characterized as a contractual view of marriage. In such instances, the wife (or husband) is entitled to alimony (for a given period of time). Homosexuals, since they are not allowed to have the rights of marriage, are susceptible to a type of abandonment without compensation. This makes a homosexual long-term relationship a higher opportunity cost (legally) than a heterosexual one, which is unfair.
Furthermore, as a Societal institution, marriage can (although often times may not) provide a more stable environment for raising a family. Of course, a male homosexual couple could not have children on their own, but adoption is always an option. For female couples, they can choose to artificially inseminate. Simply because these couples cannot produce a family in a "typical" fashion does not imply that they should not have the right to raise a family so long as they can acquire one by legal means. An obvious counterexample to a rejection of this claim would be infertile couples, or any other family that adopts or artificially inseminates. Furthermore, we allow marriage when no family is intended (or possible), such as older couples or job-oriented couples. Perhaps we should only give benefits to people who have children based in light of this societal view, but that would simply mean that "typical" marriage would not be justified from a society viewpoint, only having children (by any legal means) would.
Anyway, there is more to the debate, but I'll keep it to this for now.