The question of same-sex marriage rights is currently getting a lot of attention in this country, especially with legislation pending. As this is an issue that has a religious aspect, I have been asked several times what the Buddhist position is. This is a little tricky to answer, because there really isn't one. Buddhist monks do not perform marriages of any kind, we are in fact forbidden by our rule to do so. So the question of whether or not to perform same-sex marriages doesn't arise.
As for the more general issue involved, Buddhist ethics (at least in the Theravada or southern school in which I am ordained) does not really address the question of homosexuality. For monks, the ethical position is clear. Any kind of sexual activity, with any sort of partner, is explicitly forbidden. For the lay Buddhist, sexual ethics is laid out in the Third Precept which calls for "abstaining from sexual misconduct."
The Buddhist suttas (scriptures) are records of actual discourses, and always have a context. In the only place where the Buddha defines sexual misconduct he is speaking to a (presumably heterosexual) man, so the definition is couched in terms appropriate to that perspective. The lay man is told to abstain from sex with "unsuitable partners" defined as girls under age, women betrothed or married and women who have taken vows of religious celibacy.
This is clear, sound advice and seems to suggest that sexual misconduct is that which would disrupt existing family or love relationships. This is consonant with the general Buddhist principle that that which causes suffering for oneself or others is unethical behaviour. ("Unskillful behaviour" would be closer to the original.) There is no good reason to assume that homosexual relations which do not violate this principle should be treated differently.
Another consideration is that the Buddha often spoke about the spiritual dangers of unrestrained sensuality. This would mean, for instance, that promiscuity of any type is spiritually harmful. This is not strictly speaking an ethical issue, but one of good spiritual and emotional health. The implication in sexual matters would be that celibacy is the highest state, with monogamy a good situation for most people. Since gay people wanting to marry would presumably be monogamous and not promiscuous, this should be seen as a positive development.
To express a personal opinion, I have a hard time seeing what all the fuss is about. Opponents of allowing same-sex marriage claim to be preserving traditional marriage. However, I've never seen the argument developed to explain how allowing a minority to have a different form of marriage is any threat to the existing or potential future marriages of the majority. I respect that some religions have strong prohibitions against certain practices but I can't see the sense or justice in making such prohibitions general law. It's as if the Orthodox Jews were lobbying parliament to ban the eating of bacon.
One genuine area of concern is that of religious freedom. I would oppose strongly any attempt to force churches to do anything that runs against their own beliefs. As a Buddhist clergy person myself, I would find this precedent extremely troubling, even though in this particular case it wouldn't effect the Buddhists. The government of the day is to be commended for being careful to ensure that religious freedom is preserved; the proposed law would allow, but not require, churched to perform same-sex marriage. A grey area remains, however, in the case of civil marriages where the justice of the peace has a personal religious objection. It's not clear to me how this situation should be resolved.