The femboy is a feminized sex object, a toy, an approximation of femininity which has no power over him.
In a lot of cultures, cis-het men frequent gay and trans sex workers. It's considered no big deal. They can't get pregnant and they have no power, either over them as individuals or in society at large.
Trans women who assert our femaleness are breaching containment as far as these men are concerned.
That's part of where their transphobia comes from.