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

Oh, and moreover, they might feel threatened because they are scared of how people will judge them if those people know that they are attracted by someone trans.
Because in our society, a man has to be attracted to a women, and because in that society a trans women is in fact a man, that calls into question their masculinity (for instance) which might be "dangerous" for their reputation.