In light of Proposition 8, and the various religious arguments against gay marriage, I want to add a new perspective. Various policies regarding gay marriage in the Western world concern Sikhs. California, the home state of Prop 8, and Canada, which recently legalized same-sex marriage, contain high concentrations of the Sikh diaspora. In fact, the policies in Canada caused enough concern to generate some drama with the Sikh higher-ups (more on this later, in Sikhism and Homosexuality, Part II).
So in spite of the potential hate mail (if the wider world knew about queeristan) here is why I think that Sikhism unadulerated by cultural ideology is one of the few religions that is naturally poised to accept gay marriage.
The first point: the basic philosophy of Sikhism is that everyone is equal. It is one of the only religions that explicitly states that all sentient beings are equal. The Golden Temple, one of the most respected religious sites, has four entrances to symbolize this - anyone is welcome, regardless of their religion, color, caste, or gender. So it hardly seems in line with the religion that exceptions would be made to bar a specific group (in this case, LGBT's) from certain rights.
Secondly, the Sri Guru Granth Sahib is mum on the issue of gay marriage. All the religion promotes is a family lifestyle and marriage for all adherents because all Sikhs belong to the community and have a duty to further it. Hence, the solitary lifestyle is discouraged (no hot rogues a la the Wolverine for Sikhs). What options does this leave a gay Sikh? Either pretend to be straight and get married, or come out and still get married, but to someone of the same sex. But why even pretend to be straight? Philosophically, gay marriage and gay adoption don't seem antithetical to the Sikh religion - in fact, it seems essential for a gay Sikh to push for gay marriage and gay adoption to remain a a good Sikh.
Sikhism's foundation lies on it's tolerance and egalitarianism; it's structure promotes community, family, and marriage. Denying LGBT's the right to marry, adopt, and participate in the community doesn't seem to fit quite right.
While you wait for Part II, where we consider the realities rather than the philosophy, consider how accepting a religion will be when it has followers who have altered it's main symbol (the Khanda...click to see original form) into this: