From a Wall Street Journal interview:

Q: What did you think of Rand Paul's initial statements about the Civil Rights Act, that the government should not tell private businesses they can't discriminate? That's consistent with libertarian views, right?

A: When he made those statements, I thought to myself, "This is probably why I'm a Republican, because maybe I would not toe the (libertarian) line." I'd like to think I would have signed the civil rights bill and wouldn't have had any issues with it.

Q: You thought about this because of what Paul said?

A: Yes. As a result of his statements, I found myself engaged in discussions over just that notion. I was trying to think of examples where I would have sided with the notion that government does have a role in that capacity. Something analogous is smoking in restaurants. I was opposed to the government mandating that restaurants not allow people to smoke, believing it becomes the customer's choice whether they go in or not. But then, I thought, what about the employees? Aren't they hostage to a smoking environment, even if they don't smoke? […]

Q: Isn't the true libertarian position to support gay marriage?

A: I've taken the position that I support gay unions.

Q: Why not marriage? What's the difference?

A: Right or wrong, that's what I'm advocating.