The idea that marriage is about love is relatively modern. For much of history, it was about forging an economic unit. Going back to English common law, marriage was a completely gendered institution in which women lost their legal identity. There was a saying, "the husband and wife are one, and the one is the husband." For example, a wife could not own property or sign legal documents.
This state of being was called "coverture." Settlers brought it to the colonies, and it was part of American law until the middle of the nineteenth century. Things started to change as a result of the first wave of the women's movement and because fathers whose daughters had property or might inherit were unhappy that creditors of profligate husbands could make off with that property. In the 1970s, the U.S. Supreme Court began ruling all the remaining gender-based marriage laws as unconstitutional sex discrimination.
For most of our history, marriage could only be ended if one party committed "fault." The primary marital faults were adultery, extreme physical cruelty, and desertion. Only the "innocent" spouse could request a divorce. These rules changed in the 1970s with the beginning of no-fault divorce, one of the biggest transformations ever in marriage. No one can be forced to stay married any more.
There also used to be a "bright line" distinguishing between children born to married versus unmarried women. It's impossible to overstate the social and legal stigma. This also changed in the 1970s when the Supreme Court said it was unconstitutional to discriminate against children based on their parents' marital status.
So you have three revolutionary changes: the stigma of being unmarried and having a child lessened; it became optional to stay married; and the mandated gender roles of centuries were gone.
The Defense of Marriage Act (DOMA) was passed in 1996 because Congress feared that Hawaii would legalize same-sex marriage and that couples would flock to Hawaii then go back home, forcing everyone to recognize those marriages. DOMA also says that for the federal government's purposes, marriage can only be between a man and a woman. Federal law governs taxes, Social Security, federal benefits and obligations, and immigration. This June, the Supreme Court ruled that this latter part of DOMA is unconstitutional.
If a state allows same-sex couple to marry, the federal government must recognize those marriages.
Right now, 12 states and D.C. allow same-sex marriage. Another seven states have civil unions or domestic partnerships. Because of the change in the institution of marriage over time and the revolutionary changes in the 1970s, it makes sense to seriously question why marriage should ever be the dividing line in the law between relationships that count and those that don't.