I grew up being taught that inter-racial relationships were of a questionable moral nature. By the time I reached my mid teens, however, I pretty much dropped the idea.
In any case, I reckon Gene Roddenberry would be proud.
His point isn't entirely wrong. When you mix unique cultures, it can mean conflict. Nevertheless, as seems pretty evident, it isn't the government's right to allow or disallow marriage based on its own definition of probable outcome. Using that standard, as Nocturnus of Bored was alluding to, there are lots of hypothetically unfit couples who are so for any number of non-racially involved reasons.
In my personal opinion, rednecks aren't particularly good parents, and children who grow up without culture while surrounded by old beer cans in the overgrown grass are at a disadvantage; but that doesn't mean that I have a right to deny marriage rights to 25% or more of the U.S. population.