I don't think transwomen should have to disclose their trans status up front, but at least mentioning it before I shag them is the honest and right thing to do. You can't have a relationship built on lies.
All this actually made me thing that 20 years ago, on the British soap Coronation Street, this actually happened with Roy Cropper finding out that his then girl friend Hayley had a pickle. But he didn't chimp out and murder her in a fit of rage. He didn't take it well but when she had reassignment surgery they got back together again and the whole trans thing never got between them because it was all out in the open.
I think this is probably a more TRUE AND HONEST depiction of such things than the ridiculous idea that finding that your girlfriend has a cock makes all cis men go MURDER DEATH KILL MURDER DEATH KILL MURDER DEATH KILL. But the Tumblrinas probably would find it problematic because Hayley was played by a cis actress and it's therefore CISSPLAINING.