If they don't want the penis then why not fuck an actual female without one
I'm not sure if I should really spend the time answering this, but meh...why not.
Obviously this wouldn't apply to EVERYONE, however, if I were to venture a theory is that most transsexual women, wish to be viewed and objectified as sex objects. Now while one could of course objectify regular women this way, there is more of a psychological appeal to some one that wants to be treated as a sex object and nothing more.
You wouldn't know it by reading Getbig, but a lot of men cannot put themselves in the headspace to completely objectify a female, and even if they could, its a contrived reaction from the female, in an effort to please the male, and not a genuine response.
Of course when you have someone that feels gratification by being used in a completely carnal fashion, it creates a more psychological appeal. The penis is simply a nuisance at that point, and most likely the trade-off for this type of person.
At least this is my guess/observation.