Not known a man, to wit, carnally: See Poole "Ge 4:1 19:8 Leviticus 18:22. Keep alive for yourselves; either to sell them as slaves to others, or to use them as servants to yourselves, or to marry them, when you have prepared and instructed them.
Related Commentaries of Numbers 31:18
Numbers 31:18
18 But all the women children, that have not known a man by lying with him, keep alive for yourselves.