Thank you for the interpretative and application question, “Why do Jehovah’s Witnesses interpret “marry only in the Lord” to mean only a “Jehovah’s Witness” thus dooming sisters to being single for life due to the ratio when they could otherwise find a Christian husband?”
Although I am not a Jehovah’s Witness, may I suggest a reason why the Jehovah’s Witness interpret I Corinthians 8:39. i cite the text in the NIV, “39 A woman is bound to her husband as long as he lives. But if her husband dies, she is free to marry anyone she wishes, but he must belong to the Lord. 40 In my judgment, she is happier if she stays as she is—and I think that I too have the Spirit of God.”
The context of I Corinthians 7 is one marriage. The Apostle Paul states that the widow or widower may marriage anyone she or he wishes with the exception that the person must belong to the Lord. That person must be a believer. There is no restriction on that person’s racial or social status provided that person belongs to the Lord.
The Jehovah’s Witnesses in their interpretation is actually seeking to apply the text to their beliefs. The text doesn’t say that one must marry a Jehovah’s Witnesses, a Baptist, or a Presbyterian. The group designation of believers came later.
Thus, for the Jehovah’s Witnesses in keeping their doctrine and practices with lesser conflict mandate or recommend that a person marries another Jehovah’s Witnesses. One can foresee this avoids beliefs or practices differences between two different groups as Protestants and Catholics.
Yes, it will make it more difficult for their members to find a life-partner. Hopefully Christians who marry another Christian in their dating into engagement will resolve their differences.
-Kingston