Marry me, kingpomba?
Okay, some necessary background on Leviticus. The oft-cited Leviticus 20:13 is part of a Parshah (the weekly Torah reading) named Kedoshim. Kedoshim is especially interesting for including what is quite possibly the strictest text and the most, uh, 'hippy' text in what is the strictest Torah book. According to the Documentary Hypothesis, Leviticus (as we know it today) was largely written by Jewish priests, or Kohanim during the Babylonian exile. According to this Priestly source, God is very much interested in rituals and laws, as well as being "distant and unmerciful". Some contend that it was only meant to apply to the Priests themselves when written.
Back to Kedoshim. Kedoshim combines some of the strictest text in the Torah, together with some of the most caring. Leviticus 19:9-18.
9. When you reap the harvest of your land, you shall not fully reap the corner of your field, nor shall you gather the gleanings of your harvest.
10. And you shall not glean your vineyard, nor shall you collect the [fallen] individual grapes of your vineyard; you shall leave them for the poor and the stranger. I am the Lord, your God.
11. You shall not steal. You shall not deny falsely. You shall not lie, one man to his fellow.
12. You shall not swear falsely by My Name, thereby profaning the Name of your God. I am the Lord.
13. You shall not oppress your fellow. You shall not rob. The hired worker's wage shall not remain with you overnight until morning.
14. You shall not curse a deaf person. You shall not place a stumbling block before a blind person, and you shall fear your God. I am the Lord.
15. You shall commit no injustice in judgment; you shall not favor a poor person or respect a great man; you shall judge your fellow with righteousness.
16. You shall not go around as a gossipmonger amidst your people. You shall not stand by [the shedding of] your fellow's blood. I am the Lord. :
17. You shall not hate your brother in your heart. You shall surely rebuke your fellow, but you shall not bear a sin on his account.
18. You shall neither take revenge from nor bear a grudge against the members of your people; you shall love your neighbor as yourself. I am the Lord.
And from there on, most of the Parshah are the commandments which people like to recite - no shaving, no tattoos, no having it up the butt with another guy. So let's skip to the interesting bits - Leviticus 20:8-22.
8. And you shall observe My statutes and fulfill them. I am the Lord, Who sanctifies you.
9. For any man who curses his father or his mother shall be put to death; he has cursed his father or his mother; his blood is upon himself.
10. And a man who commits adultery with [another] man's wife, committing adultery with the wife of his fellow the adulterer and the adulteress shall surely be put to death.
11. And a man who lies with his father's wife has uncovered his father's nakedness: both of them shall surely be put to death; their blood is upon themselves.
12. And a man who lies with his daughter in law both of them shall surely be put to death; they have committed a depravity; their blood is upon themselves.
13. And a man who lies with a male as one would with a woman both of them have committed an abomination; they shall surely be put to death; their blood is upon themselves.
14. And a man who takes a woman and her mother it is evil counsel. They shall burn him and them in fire, and there shall be no evil counsel in your midst.
15. And a man who lies with an animal, shall surely be put to death, and you shall kill the animal.
16. And a woman who comes close to any animal so that it will mate with her you shall kill the woman and the animal; they shall surely be put to death; their blood is upon themselves.
17. And a man who takes his sister, whether his father's daughter or his mother's daughter, and he sees her nakedness, and she sees his nakedness it is a disgraceful act, and they shall be cut off before the eyes of the members of their people; he uncovered his sister's nakedness; he shall bear his sin.
18. And a man who lies with a woman who has a flow, and he uncovers her nakedness he has bared her fountain, and she has uncovered the fountain of her blood. Both of them shall be cut off from the midst of their people.
19. And you shall not uncover the nakedness of your mother's sister or your father's sister, for he would be baring his close relative; they shall bear their sin.
20. And a man who lies with his aunt he has uncovered his uncle's nakedness; they shall bear their transgression; they shall die childless.
21. And a man who takes his brother's wife it is a repulsive act; he has uncovered his brother's nakedness; they shall be childless.
22. And you shall observe all My statutes and all My ordinances, and fulfill them, then the Land, to which I am bringing you to dwell therein, will not vomit you out.
So those are a whole lot of verses dealing with sexual misconduct, with the punishment for (almost) every case being killing the participants. Exile is also an option for some. These include having sex while on your period, adultery, bestiality, many different cases of incest, and two guys going at it.
Now, here is the interesting bit. Some say that these are simply cases of sexual misconduct, and all are equally bad. Others contend that these are not allowed because usually there is some sort of abuse going on, with the male being in an authoritative position over the female. Same-sex relationships at the time were usually a master and his servant, and therefore are also disallowed.
Now, as for how this all relates to the same-sex marriage argument. We allow people who commit adultery to later marry their lover, right? But the prohibiting verses are almost identical! Compare verses 10 and 13. And I don't see anyone suggesting we should go around punishing women who have sex on their periods... So why are we taking this one verse and glorifying it, holding it to be so important, while ignoring the other ones?
My own personal position? I don't see anything morally wrong with homosexuality or homosexual conduct, but even if I did, it's still not any of one's business. There is no rational reason for a State to disallow same-sex marriages. In order to deny equal treatment to a group of people, you need a very strong reason to do so (not Constitutionally in Australia, but things should certainly still operate this way).