Not Always Wrong
Mark Regnerus is a sociologist at the University of Texas–Austin and coauthor of Premarital Sex in America: How Young Americans Meet, Mate, and Think about Marrying.
No, interfaith marriage is not always wrong. But yes, avoiding being "unequally yoked" is an excellent biblical principle. The question itself requires more excavation.
Paul advised the Christians at Corinth to avoid entering significant relationships, such as marriage, with unbelievers. There you have it: Don't marry an unbeliever—that is, someone who doesn't share the basics of Christian doctrine and practice. As those who have been there can attest, raising the next generation of Christians is simply tougher when one parent is dragging his heels or openly balking. It can be done. I've seen praiseworthy spouses watch their mates come around to faith. But that's a rare outcome.
Genuine interfaith marriage is a challenge I don't recommend. But as marriage has shifted in purpose over time, many Christians have added layers of meaning onto Paul's wise command. "Unequally yoked" has evolved into a graded criterion for an optimal mate rather than a simple test for an acceptable one. This is a problem.
Why? Spiritual maturity is not equally distributed among men and women in the peak marrying years. Quality survey data reveal only two serious, churchgoing evangelical men for every three comparable women. Thus, one out of every three evangelical women is not in a position to marry a man who's her "spiritual equal," let alone "head."
This elevated standard now translates—for women, at least—to something like this: "Find that uncommon man ...