L, the difference is that "marriage" in this country (and most others) comes with a whole host of privileges and powers. Included among them is the ability to have your partner with you in the hospital in case of serious injury, health care familial extension, financial benefits of tax breaks and survivorship rights in the case of a death.
Personally, I think that all of those privileges currently associated with marriage should be delegated to a "civil union" by the government, and that "marriage" can be something a religion can do if it likes purely for ceremonial and religious reasons.
Now as to most religions hating homosexuality, you'd actually be wrong. Going by the number of religions, rather than the number of their adherents, there are by far greater number of religious that are ambiguous on their stance towards homosexuality. Remember that Christianity is a relatively minor number of sects compared to the diversity of the rest of the world, although those sects have a large number of members in North America.