I think abortion should be left to the states, but I think marriage should be federally mandated: one man and one woman.
That's exactly Romney's stance. It's not hypocritical either (on first blush many people think that). Romney layed out the reasoning very well on his Charlie Rose interview (search for it at Google Videos). Essentially, marriage is a status that allows for various entitlements and familial units that, necessarily, would have to be recognized by other states.