Steve, you said "women can get pregnant and men can't" isn't sexist.
What do you think of Thomas Beatie? He's a transsexual male (FTM) who is married to a woman who had to undergo a hysterectomy. When the time came for them to start with children, they decided he carry the fetus instead of choosing adoption.
Is he not pregnant? Or is he not a man? If the latter, what determines whether one is a man?
Personally, I think the categories of 'man' and 'woman' are a little too vague to be certain about anything. Some women are infertile, or had their tubes tied, and will never get pregnant. Not women? Some people are intersex. Are they not the gender they think they are? How 'bout XXY (2 Xes, therefore woman AND Y, therefore man) and X0 (no 2 Xes, therefore not woman, and no Y, therefore not man) genotypes?