There are guys who seem turned on by the castratrix (penectomix?) doing wasteful things with her prize.
To stomp on it, feed it to a dog, flush it down a toilet, send it through the garbage disposal, etc.
To me, that just nullifies the whole "desire" component. She never really wanted it in the first place, if all she does is just throw it away.
So, what she does with it once she has it, is key to making the fantasy hot for me.
For one thing, I've noticed that the image of a hot chick playing with a severed thingy is quite arosing to me.
And if she does eventually get tired of it and wants to get rid of it, the sexiest thing she can do is literally gobble it up.
It's not that cannibalism, itself, is sexy to me. That's just one person pigging out on another, as far as I'm concerned, so it does nothing for me.
I guess it is another perversion of basic straight sex drive. It still is, after all, the he part entering the she body, isn't it. Just another orafice.
Fans of vampires say that the concept is sexy to them because the vampire's bite is similarly disguised intercourse. His fangs are what do the penetrating.

There you have it -- my self-analysis of why this fantasy gets me hot and bothered, whether I'm dead on or way off.