I think it's less a subversion of that trope and more a continuation of a "nooo, women are corrupting" idea (think about men in a monastery, though it may also just resonate more with me because I'm Welsh and Welsh women were particularly demonised as being immoral and such, a century and a half ago). Ursula Le Guin has it in the Earthsea books, and deconstructs it much more than Stewart does. Stewart's final note says that she included that aspect of Merlin's power because virginity is traditionally linked to power throughout history.