It's a proud day in Vermont, which today became the fourth state -- following Massachusetts, Connecticut and Iowa -- to allow same-sex couples to marry. In a historic vote, the Legislature overrode the governor's veto of a bill that allows the couples to marry. (Don't miss this tear-jerking victory photo of advocates in Vermont expressing their joy at the good news.)

Vermont is the first state to enact marriage equality for same-sex couples entirely through the legislative process. Incidentally, twice the California State Assembly has passed similar marriage legislation, in 2005 and 2007, but both times Gov. Arnold Schwarzenegger vetoed it.

But the significance of the news in Vermont today goes beyond the legislative triumph, according to Evan Wolfson, executive director of Freedom to Marry, and author of "Why Marriage Matters." Vermont was the very first state to offer civil unions for gays and lesbians, which it did in 2000. "They've now said, 'It's not good enough,' and that's very important for the country to hear," says Wolfson. "Civil union is not the same as marriage, and it's not a substitute for the freedom to marry. It doesn't provide the security, the safety net, the legal and economic protections, or the cherished meaning that comes only with marriage."

So, which state will likely be the next to allow gay and lesbian couples to tie the knot? Possibilities include New Jersey, New York, New Hampshire and Maine. The District of Columbia may soon go to the altar, too.

In New Hampshire, the House has already passed a bill that would legalize same-sex nuptials, and it's now pending in the Senate, where it will likely be voted on in the next few weeks. Both New York and New Jersey have similar bills under consideration, which the governors in those states have pledged to sign if the bills make it to their desks. In Maine, another same-sex marriage bill was introduced this year, and committee hearings are expected in the next few weeks.