It's not that the male villains can't be or aren't portrayed that way. It's that they're also portrayed as Evil Towers of Ominousness, and any sex appeal they have in that state is entirely apart from the writhing sensuality of the female villains.

For the male villains, sexuality is an accessory. Some pick it up casually, some base their entire image or purpose around it. For the female villains, sexuality is built into their entire premise. It's having skin. They'll do different things with it, and for some it's a much bigger part of their image than for others, but there are very few without it. Isard might be one, I suppose. Advances on Fel notwithstanding.