So here, the idea is to explain why cis people might feel threatened by the very existence of trans people.
Because of oppositional sexism, you can't really be feminine if your body is a "boy's", and the whole idea of their masculinity is based on that, that they can't be woman because of their body. Being trans makes them call their whole concept of masculinity or feminity, their whole identity to some extent. The legend says that if you're lesbian or gay while being trans, their mind explodes !
The whole idea of oppositional sexism is, also, that the border between the two is impossible to cross, so obviously, when you do...
#WhippingGirl

@Mawloc if you define masculinity or femininity in a positive sense ("who I am", not "who I am not") - this theory makes no sense.

@saper The whole point of the concept of oppositional sexism is that is one gender is something, the other is the opposite. If you say I am, you imply "They are not", that way, it doesn't matter how you define masculinity or feminity. And actually, it seems that our society is working that way, considering that feminity and masculinity are in fact opposite and therefore exclusive.