About Kids Health has an excellent animated explanation of sexual differentiation during fetal development. Click here to navigate through chromosomal, gonadal, internal sex organs, and external genitalia. The part on external genitalia, here, is especially useful for demonstrating that, though we use the term “opposite sexes” and think of our bodies as like opposing puzzle pieces, our external genitalia is made from the same tissue and so makes sense to expect that our sexual sensations are not so different.
Further, the illustration might be useful for discussing intersexuality. It doesn’t seem so unusual when you can watch the bodies develop. A little bit less development one way, or a little bit more the other, and a body that does not fit the gender binary is made.
Also in intersexuality, don’t miss the phall-o-meter!