My understanding is that the TOB is about difference in unity, complementarity in unity.  Of course there is a difference between man and woman (what else is new?), but the difference is not ontological and does not cancel the original unity in one and the same human nature, one and the same flesh, fully homogeneous in "their whole being" even though in two different "incarnations."  See, for example, TOB 8:1, 8:4.

If you prefer to think in terms of biology at any level, keep in mind that male is XX and female is XY, but we share the same X, and the XY is a differentiation from the XX default *after* conception.  Biologically, one and the same human nature, same flesh.  Psychologically, it is well known that there is man in woman, and there is woman in man; again, one and the same human nature, same flesh.  Theologically, one and the same same human nature, same flesh.  

In the TOB, the human body is always male *or* female, but the human person is always male *and* female... :-)