Even if we applied it to marriage, in the first century, all that was necessary for being a Christian was belief that Jesus is the Christ and the only way to gain access to God the Father. Not belonging to a specific group of Christians who bear a specific organizational brand name. So if we actually used this concept in terms of what the Bible actually says, Christians would just be limited to marrying other Christians. Sounds like a good deal! A lot more options that way.
Furthermore, even if marrying a non-Christian was forbidden, does the New Testament contain any punitive measures to be doled out if someone chose to do so? No. That being said, it seems more logical to let people make their own decision on that. After all, Paul said women should not speak in the congregation, but they sure speak during JW meetings. How far do we go with turning Paul's letters into a new Law? It can get to a rather murky place, if you ask me.
-sd-7