I'm going to have to disagree with both
Shara and
Djoser about some things.
Shara, it's a never a good idea to make a generalization about everyone in a category (all men, in this case). I, for one, do not feel a need for a bigger penis. I'm satisfied with what I have. I suspect other men feel the same way.
Djoser,
Futanari has very little to do with biological definitions of what is a male or female. It has much to do with fantasy (as you alluded to). While it's true (in real life) that there are men who have taken the necessary hormones to gain feminine characteristics while keeping their penis, Futanari refers to DRAWINGS of otherwise anatomically correct
women who "happen" to have a penis (with or without a scrotum). Usually they
also have a
vulva at the same time! Having said that, ..I don't understand the appeal, either. I've found art that depicts a "chick with a dick" and admired the woman's figure, while wishing the penis wasn't there.

I've even altered a drawing or two to make that wish come true.

What I'm really trying to say is that there are more things than the presence or absence of a penis that determine a male or female via anatomy. Even if we "ignore" both primary and secondary characteristics (meaning breasts), there are differences in body shape between men and women. Hips are the most obvious, but other areas of the body give away gender, too.