Tax incentives, ability to act on the other's behalf in case of emergency (ie, say you're in a bad car accident together and one's recommended to have X procedure for better healing. A husband/wife can okay the procedure, boyfriend/girlfriend cannot). Another scenario: (cont)

(C. 1) If you own property "together" (one person holds title but both reside on property), in the case of death, spouse gets property (most states). Even in divorce, you're still entitled to half of property value/half of equity accrued during marriage (state depending).

(C. 2) for same sex couples, marriage is especially important if children are involved. It makes adoption easier and allows you to act on behalf of your child, regardless of biologically related (ie partner A is bio parent, if married, partner B can act for child).