Hair cleanly parted down the center is one of the most sexually suggestive ways someone can wear their hair IMO. The vertical line down the middle immediately brings a woman's cleavage or butt cleft to mind, and of course it can only be done with long, feminine hair anyway. The center-part is the foundation for pigtails and double buns, which are 'very' sexual styles in their own right. But people are allowed to show a center-part off without acting like it's sexual, and I think that's what makes it so hot. Everyone has to pretend that the sexual signaling is innocent or unintended, while people honestly know that it's not. Even folks without a full blown hair-fetish know this at some level, because you almost never see old, gray-haired women wear their hair like this.