Here's the most relevant reference I could find on the topic:
*** w82 3/15 p. 31 Questions from Readers ***
Finally, in the Christian Greek Scriptures God repeated the counsel: Do not marry someone who is not serving the Lord. The inspired counsel was not, ‘If you find a clean, decent person, it is permissible to court and marry that individual with the hope that he or she may eventually become a Christian.’ Rather, God’s Word clearly says: "Do not become unevenly yoked with unbelievers." (2 Corinthians 6:14) To marry someone who is not already a baptized Christian would be to disregard that serious counsel.
Because Jehovah’s Witnesses as a people view this counsel as wise and serious, they do not want to contribute to anyone’s going contrary to it. For example, if because of weakness a spiritual brother or sister began to court or date a person who is not a witness of Jehovah, the others in the congregation would hardly want to encourage that by socializing with the unbeliever. They agree with the Bible that the non-Christian is not a good associate. (1 Corinthians 15:33) But they should continue to display interest in their brother or sister. They might be able to offer tactful and encouraging admonition to help the straying Christian to avoid the sad path that Solomon followed.—Compare 2 Thessalonians 3:14, 15.
But what if a Witness planned to disregard God’s advice and to marry someone who was not a baptized Witness? Unless there was some exceptional reason, brothers in the congregation would not want to solemnize such an unequal yoking. Nor would the Kingdom Hall be available for the wedding. It is available for marriages of two baptized Christians who are marrying "only in the Lord." Or it might sometimes be used by two persons who are regularly serving God as part of the congregation and who will soon be baptized. By not allowing the Kingdom Hall to be used by a Witness who plans to ‘become unevenly yoked with an unbeliever,’ the congregation elders can underscore the seriousness of God’s counsel to marry "only in the Lord."