This strikes me as a pertinent issue, because Mitt Romney has set himself up as a guardian of the institution of civil marriage. He certainly wants to prevent me from legally marrying my fiance. For a long time, inter-racial marriages were also taboo in the LDS church, and you can read a lively pro-Mormon discussion of the issue here. Currently, according to this site, the LDS policy is to discourage interracial marriage, but not to forbid it. Money quote from former Mormon president, Spencer Kimball:
"The interrace marriage problem is not one of inferiority or superiority. It may be that your son is better educated and may be superior in his culture, and yet it may be on the other hand that she is superior to him. It is a matter of backgrounds. The difficulties and hazards of marriage are greatly increased where backgrounds are different. For a wealthy person to marry a pauper promises difficulties. For an ignoramus to marry one with a doctor's degree promises difficulties, heartaches, misunderstandings, and broken marriages.
When one considers marriage, it should be an unselfish thing, but there is not much selflessness when two people of different races plan marriage. They must be thinking selfishly of themselves. They certainly are not considering the problems that will beset each other and that will beset their children.
If your son thinks he loves this girl, he would not want to inflict upon her loneliness and unhappiness; and if he thinks that his affection for her will solve all her problems, he should do some more mature thinking.
We are unanimous, all of the Brethren, in feeling and recommending that Indians marry Indians, and Mexicans marry Mexicans; the Chinese marry Chinese and the Japanese marry Japanese; that the Caucasians marry the Caucasians, and the Arabs marry Arabs."