He comes from a planet where children are genderless and become male or female during puberty. At the right age, kids start receiving hormone treatment to make sure they become the "right" sex (otherwise the process would be random) - only the oldest one is allowed to become male in order to preserve 1:5 male:female gender ratio. Although the majority of the society are women, men are the priviledged ones. Frol has already started receiving female hormones, which terrifies him, because he sees becoming a women as being senteced to life where his only purpose is getting married and having children (with some old guy who already has 8 wives, no less).
As to Knu, his species works similiarly. "Menir" and vidmenir" refer to those who haven't gone through puberty yet and those who have but didn't become male or female.