First of all, I think that if we really want to be fair, marriage should be banned for heterosexuals, rather than allowed for homosexuals
Secondly, no, I don’t think churches should allow it, because it goes against the rules of the particular religion, and by changing these rules, you change the religion (I think the same about women priests for example). But of course it’s fine if you decide you’re going to modernise your religion and have a new sect, with the same basic ideas as Christianity, but changes to make it more relevant to modern society. In that sense, religion can evolve and move forward with the times, if it so chooses.
Thirdly, as far as the practical concerns go, I believe that if two (or more) people choose to live together as a family, they should be legally allowed to do so, regardless of their sexual preferences. But this should be done through civil weddings, and not religious rituals, which are meaningless nowadays anyway. This travesty of men dressing up as brides and women dressing up as grooms should stop. But at the same time someone who’s been your life partner should be entitled to a pension and inheritance, and any other social benefits other married couples enjoy. And the law should help that happen.