From one very narrow and simplistic perspective, the Old Testament is clear cut on its stance concerning homosexuality. Leviticus 20:13 says, in so many words, that sexual bonding between two men is "an abomination" and for anti-gay activists this has been enough. From an educated standpoint, the popular interpretation of this passage is inadequate. Though I cannot speak for New Testament scripture on the topic, I would like to offer my own theological argument for why Leviticus 20:13 should not be considered presently valid.
The biggest fallacy of biblical exegesis is the assumption that the bible is a static document, that all statements within it are to be considered true in perpetuity. This has led many to point out that the bible is often self-contradictory, and they're right. I can think of another list of rules that is also self-contradictory- The Constitution of the United States of America. After all, isn't that what an amendment is, a contradiction of an earlier statement within the same text? Whether you believe it is literally true or merely a compilation of cultural myth, the bible is clearly a linear story. It is no more logical to say that all rules mentioned in the bible are forever true than it is to say that Moses is simultaneously a baby on the Nile and a dying man on Mt. Nebo.
There are also plenty of instances in the Old Testament of existing rules being overturned upon review. Take the case of the three daughters of Zelophehad. When the man died, his daughters (being his only living kin) petitioned to claim his property as an inheritance despite the existing law that stated only male heirs could claim familial property. The final ruling overturned the existing law and the daughters each claimed an equal portion of their family estate. This is in clear contradiction of previous inheritance claims such as the dispute between Jacob and Esau. Plainly, the law changed when a logical argument against it was tried by impartial judges.
Though there is no instance of the ruling in Leviticus 20:13 being challenged in scripture, the text gives us precedence for the challenge of any and all laws, with the possible exclusion of the Ten Commandments. Considering that no real reason for the law in Leviticus 20:13 is present, a moral argument against its ruling can be made.
It is my contention that in prohibiting homosexual unions, Leviticus 20:13 results in a much greater sin. According to several debates in the Talmud between the great sages (such as in tractate Pesachim), it is unacceptable to follow a rule that directly results in greater harm than the rule itself prevents. Given the conditions of our modern society, prohibiting same sex unions results in several different kinds of harm. For instance, if we allowed same sex couples to adopt orphaned children and thus be recognized as a family unit, we would have fewer children growing up without guidance and care. Is it not a greater sin to leave a child orphaned than to engage in a same sex union that does not clearly harm anyone?
But beyond the argument for harm vs. benefit is the fact that the rule in Leviticus 20:13 is meant for people in a dramatically different circumstance than our own society. The rule appears alongside a series of laws about maintaining purity, not mixing things of different qualities. These are laws intended for a very small, fledgling nation that was under the constant threat of assimilation into other cultures. The purity laws are safeguards against foreign domination, thus no longer relevant in the 21st century just as all biblical rules concerning animal sacrifice have long been considered ill-suited for righteous people.
As there is no scriptural evidence for the harm of same sex unions and a clear imbalance in the harm to benefit ratio of their prohibition, I believe that it is theologically sound to suggest that the law stated in Leviticus 20:13 is no longer valid and it should not be considered a measure of sin unless someone can produce a reasoned argument in support of the initial ruling.