I have two objections to it:
1) Fantasy worlds not based strictly on historical molds have fewer excuses. If the world accepts homosexuality and says that women can, for example, be queens and businesswomen and mages, why the fuck do they have to be virgins in other professions? You'd think female soldiers would be possible- adjusted for weapon size and physical strength- the same way female mages and rulers and businesswomen are possible, and there's no need for silly rules about sexuality.
2) The emphasis on virginity and lack of children in worlds set up this way pretty much guarantees that the focus will be on young women, or teenagers. What about a widow who has borne a child, so definitely isn't a virgin, but still wants to enter a male profession? Why should she be forbidden (by the author) because of the silly, silly emphasis on virginity?
And possibly also:
3) The author lets men do whatever they like, sexually, but punishes her female characters for expression of it- as if every single sexual act ever done would result in pregnancy. I would think that use of hands and tongue could satisfy, but many authors prudishly forbid women even that.