Melissa has a post
about the real-world consequences of "letting the states decide" on issues like homosexual marriage. She references an article
about a Massachusetts judge ruling that 170 marriages between citizens of New York are in fact valid. The ruling was necessary because Massachusetts law states that same-sex unions cannot be recognized if the couple's state of residence disallows them. New York doesn't prohibit same-sex marriage, so those marriages are valid.
Of course, no same-sex couple married in Massachusetts is going to get federal recognition of their marriage. In all areas where federal law trumps state law, their relationship has no legal standing.
Apart from how things are turning into a mess nationally, with laws in some directly opposing laws in others, there is a real problem with the idea that the states should address this issue. This argument is also the one being made about abortion, usually in the context of people advocating for the Supreme Court to overturn Roe vs. Wade
It's also the argument made about slavery and the supposed rightness of the Confederate cause during the Civil War. States' Rights
they cry, no matter the issue, so long as kicking it to the states will allow them to form fortresses of oppression within this land of freedom and liberty.
I've no problem with pushing issues at the state level. But marriage is obviously not an issue that can be adequately addressed only at that level. Aside from the federal benefits/costs of legal recognition or lack of same for same-sex relationships, there is the fact that heterosexual couples married in one state have no problem with their marriage being recognized in another, while same-sex couples find themselves faced with the choice of either staying where they are or living in a place that refuses to treat them, legally and socially, as what they are.
Before the Civil War, things were largely the same for freed slaves. In one state they were free, but cross the border to another and they would be rounded up and put to work on a plantation or killed, no matter what standing they had in the state next door. When the pro-slavery elements of American society saw that they were no longer going to be able to bully Congress and the White House, they declared it an issue of states' rights, and when that failed to get them what they wanted, they initiated a bloody war.
It's interesting to note that abortion is treated much the same way. Despite the current makeup of the Supreme Court, anti-abortionists know they're losing the battle on this, so they want to throw it back to the states. Of course, when they have a chance to ban a particular abortion procedure using Congress, they do it as quickly as possible, because when they say "states' rights" they don't mean that states should have the right to oppose their point of view.
Again, just like with the fights over slavery, when the southern states demanded that as territories were added to the USA they would alternate between free states and slave states so that representation in Congress, etc., would remain constant. This they demanded no matter the wishes of the people in the newly formed territories.
No matter how much time has passed, it seems we will never be free of the pernicious legacy of the Civil War.