The holy books of the Abrahamic religions all seem to proscribe homosexuality. Why did this become such an important characteristic of these religions?
Since these religions developed during the time of the Roman republican and empire, and the Romans had fairly accepting attitudes towards homosexuality, is it probable that the religions were reacting against Roman practices?
The tenets of Judaism were formed before the Roman Empire and Islamic & Christian beliefs about many "sinful or unclean" acts derive from Jewish belief. If you look at what the Jewish people were (nomadic, tribal, herders in an arid environment) most of the Levitican prohibitions make sense.
As far as why Greco/Roman & Judeo/Christian attitudes toward homosexuality differ, much of that has to do with with the differences between a religion who's God rewards and punishes based on your moral behavior as opposed to having gods who reward and punish based upon how well you worship and make the proper sacrifices.
Religiously it comes down to why we have sex. God intended mankind to "be fruitful and multiply", that was the purpose of sexual intercourse; therefore, any sexual practice which deliberately prevented the possibility of pregnancy (homosexuality, birth control, etc) went against God's will and was "unnatural".
Islam and Judaism both arose from cultures with a strong emphasis on family; men were expected to marry, have children, and extend the family's influence, property, and wealth. A man in an exclusively homosexual relationship was not capable of doing this and posed a danger to his family. A married man in a homosexual relationship could be considered akin to adultery in that his concerns are being distracted from his lawful family. Lesbianism is considered a lesser evil because there are few social repercussions from it. A woman's purpose was to bear children and take care of the household. The woman would marry and have intercourse with her husband regardless of her sexual preference, lesbian acts carried no chance of bearing another man's child, and lesbianism between wives could strengthen the family bond.
I can't speak for Islam, but in Jewish & Christian belief there is absolutely nothing sinful, unclean, whatever you want to call it about being a homosexual; loving or being sexually attracted to a member of your sex. The prohibition is against homosexual acts. Because religiously it was unnatural and against God's intentions and socially it disrupted the family structure.