The court seems ready to strike down the federal Defense of Marriage Act, while ruling quite narrowly on California’s Proposition 8, allowing a lower-court decision to stand. Such an outcome would make gay marriage legal in California without deciding whether state bans on same-sex marriage are constitutional.

And that would allow more of what we’ve seen up to now: a growing number of liberal blue states moving to legalize gay marriage, and a growing number of conservative red states enacting bans.

But there will be one big difference: Gays who live in states that allow gay marriage may have an array of federal privileges unavailable to those living in states that ban such marriages. And that raises complex questions.

What happens to two gay men who marry in New York and then move to Salt Lake City? Will they still be married? If they have children, will the kids have two parents under Utah law? And will their federal benefits, such as survivors’ Social Security benefits, travel with them, even though they’ve moved to a state where their marriage isn’t valid? Will they file their federal tax returns jointly but state returns separately? And don’t even think about the issue of divorce.