As far as I'm aware, the KJV's translation is the most literal, and it preserves the ambiguity that is in the original, as to what "his virgin" means. Hypotheses I've seen are:
1. His virgin daughter/ward. It is a stretch to suggest that the interpretation is condoning incest; it is far more likely, rather, that it was urging against fathers refusing to marry off their daughters because they wanted the daughters to stay celibate for spiritual reasons.
2. His engaged virgin. It is suggested that this would refer to couples who were engaged, but who indefinitely postponed marriage for the sake of spiritual celibacy. (cf Rutherford's encouragement of this step in the book Children.)
3. His virginity. The WTS are not the only ones to suggest this reading, although I do not believe it has wide support.
4. His spiritual wife. Some scholars try to relate this to the later Christian practice of spiritual marriage, where couples would marry but abstain from sex. (cf 1 Cor 7:1,5) From what I saw, most scholars seem to feel that this interpretation is an anachronism; although of course, that depends on the dating of the passage.
Sorry that I don't have references for any of the above; it's been quite some time since I researched this. Like peacefulpete, I too would be very eager to see what Narkissos has to say on this topic.