"While being married to a non-Christian often makes it more difficult for a Christian woman to remain faithful, there is no requirement that Christians -- male or female -- must only marry a Christian."
But what about: I Corinthians 6:15-20; I Corinthians 7:39; and II Corinthians 6:14-18? I could find much more reasoning, but that is a start.
But really, I am surprised to see this statement considering you taught the contrary in Growing Up in the Lord: A Study for Teenage Boys:
"Can you understand now why God forbade the Israelites to marry non-Jewish people (Nehemiah 13:23-27)? Those marriages led more Israelites away from God than any other factor we can find in the Old Testament. For a similar reason, God told the Corinthian widows, in I Corinthians 7:39, to only marry another Christian. If it makes sense the second time around, doesn't it make good advice the first time? Don't become unequally yoked to a nonbeliever (II Corinthians 6:14-18)."
There is a difference between a requirement and a preference. A requirement must be followed. To not do so would be violating God's will. So is there a requirement that a Christian must marry a Christian?
"Wives, likewise, be submissive to your own husbands, that even if some do not obey the word, they, without a word, may be won by the conduct of their wives, when they observe your chaste conduct accompanied by fear" (I Peter 3:1-2).
Notice that Peter's instructions to Christian women is predicated on the fact that some were married to unbelievers. This verse alone proves that it isn't a sin to be married to an unbeliever. But Paul also argues that a marriage to an unbeliever is to remain intact.
"But to the rest I, not the Lord, say: If any brother has a wife who does not believe, and she is willing to live with him, let him not divorce her. And a woman who has a husband who does not believe, if he is willing to live with her, let her not divorce him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy" (I Corinthians 7:12-14).
I don't generally advise a Christian to marry an unbeliever, in the answer you cited, it was a case of an older unmarried Christian woman who had been unsuccessful in finding a mate. While it is far better to marry a Christian, if none are available and one is a strong Christian willing to deal with the extra difficulties of being married to an unbeliever, then there is no hard rule against such marriages.
I Corinthians 6:15-20 is discussing sexual sins and isn't applicable to whether a person should or should not marry an unbeliever. See "Immorality" for more details.
I Corinthians 7:39 is discussing whether widows should remarry in light of the present distress. Again, it isn't directly applicable to the situation. Since writing the Growing Up in the Lord books, I've come to learn there is some debate over to what the prepositional phrase "only in the Lord" applies. The more I've studied it, the more I'm leaning that is should apply to the verb and not the object of the statement as frequently done. See "Only in the Lord" and "Because of the Present Distress" for more details. Because of this, I'll probably modify my argument a bit in the Growing Up in the Lord books when do another edition.
II Corinthians 6:14-7:1 is probably the strongest argument showing the inadvisability of tying yourself to an unbeliever. It ought to be avoided because of the dangers it presents. Paul is not specifically talking about marriage in this passage, though I think the principle applies to any consideration of marriage. The reason it is not directly applicable is because Paul cites Isaiah 52:11 which says to severe inappropriate ties -- the opposite of what Paul stated specifically about marriage to an unbeliever in I Corinthians 7:12-14.
Marriage to an unbeliever is not advisable as it is frought with many dangers. It is much better to marry a Christian. However, it is not a sin for a Christian to be married to an unbeliever; it is just a poor choice.