There was a time, not that long ago, when this was illegal, too. It was only in 1967 that the U.S. Supreme Court legalised inter racial marriage.
Yes, yes, I know. Those who oppose marriage equality have sincerely held views.
So did those people who opposed inter-racial marriage. Didn't, and doesn't, make their views anymore right.
Inter racial marriage was the subject of the wonderful film "Guess Who's Coming to Dinner" made in 1967.
The final speech by Spencer Tracy was a cracker.
"As for you two and the problems you're going to have, they seem almost unimaginable. But you'll have no problem with me..... But you do know - I'm sure you know - what you're up against. There'll be a hundred million people right here in this country who'll be shocked and offended and appalled at the two of you. And the two of you will just have to ride that out. Maybe every day for the rest of your lives.
You can try to ignore those people or you can feel sorry for them and for their prejudices and their bigotry and their blind hatreds and stupid fears. But where necessary, you'll just have to cling tight to each other and say screw all those people!
Anybody could make a case, and a hell of a good case, against your getting married. The arguments are so obvious that nobody has to make them. But you're two wonderful people who happened to fall in love and happen to have a pigmentation problem. And I think that now, no matter what kind of a case some bastard could make against your getting married, there would be only one thing worse. And that would be if - knowing what you two are, knowing what you two have, and knowing what you two feel - you didn't get married."
The specifics are different, but the sentiments are exactly the same.