I don't know that I would call the Net Bible liberal. The directors of the team were all Dallas Theological Seminary profs, including Daniel Wallace, who is probably the main exponent of the translation among scholars.
I certainly would agree that there are concessions to liberal thinking, such as the rendering in Isaiah 7:14 of ‛almâh (עלמה), which as has been pointed out is rendered as "young woman" rather than "virgin." That goes right along with liberal versions going back to the RSV, and it was protested back in that day by many fundamentalists.
I've done my own research on alma. It occurs only 7 times in the OT, and here there are:
Ge 24:43 Behold, I stand by the well of water; and it shall come to pass, that when the virgin cometh forth to draw [water], and I say to her, Give me, I pray thee, a little water of thy pitcher to drink;
Ex 2:8 And Pharaoh's daughter said to her, Go. And the maid went and called the child's mother.
Ps 68:25 The singers went before, the players on instruments [followed] after; among [them were] the damsels playing with timbrels.
Pr 30:19 The way of an eagle in the air; the way of a serpent upon a rock; the way of a ship in the midst of the sea; and the way of a man with a maid. {midst: Heb. heart}
Song 1:3 Because of the savour of thy good ointments thy name [is as] ointment poured forth, therefore do the virgins love thee.
Song 6:8 There are threescore queens, and fourscore concubines, and virgins without number.
Isa 7:14 Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. {shall call: or, thou, O virgin, shalt call}
As has been pointed out, there are usages which are provably "virgin." However, just looking at the list you can tell that there is not a single usage where the meaning is provably not referring to a virgin.
