Women can be toxic, since they're human, but it's not their femininity per se that's toxic. The reason masculinity can be toxic and femininity can't is that.....<drum roll> masculinity and femininity are two different things having different qualities to them. The primary qualities of femininity are that it's soft, gentle, nurturing, etc., so it's pretty much the opposite of toxic. Of course it's worth mentioning that masculinity, probably including toxic masculinity, is engendered through sexual selection just like femininity is, so women are to blame too for encouraging it. How often have you seen women complain that they go for all the "bad boys" who treat them like shit?