One of the reasons that so many people are pushing for the legalization of gay marriage is that marriage is more than just a spiritual or love-based bond. It is also a legal contract. A legal contract that provides certain benefits:
2) Marriage can also result in lower taxes.
3) Sharing a single health insurance plan typically generates savings.
4) Spouses don't pay estate tax.
5) Gifts between spouses are not subject to gift tax.
6) Marriage can offer financial protections in the case of divorce.
7) Social Security benefits go to the surviving spouse.
8) Property is more easily shared between married partners.
The article above has more details on these legal benefits provided by marriage, currently not available to unmarried gay couples.