We're lucky we're not anglerfish. (For those not biology geeks: the male anglerfish is a fraction the size of the female, and early in life he attaches to her skin and all his organs except the gonads atrophy. He becomes part of her body, his only function to emit sperm when she lays eggs.) It would be hard for a feminist anglerfish to argue for equal roles in society. Males can't even eat on their own.
Human males and females can both eat, are roughly the same size, and even come in the same range of colors. We got off pretty easy on the dimorphism front.
But not completely. For all we've got in common, there are some differences that can't be denied. Like, the babies only come out of one sex. And there may be strong women and weak men but for practical purposes men get a lot more muscle a lot easier. As for intellectual differences--there's no reason one sex couldn't be on average smarter. I'm not saying it's so, it's definitely not pronounced, but it's possible there's a statistically significant gap. The ideological repugnance of the idea that men are naturally better at abstract reasoning doesn't translate to physical impossibility.
(Yeah, says a lady.)
What I'm getting at is that it's useless for feminism to be strictly descriptive--to say that men and women are the same may not be true. It's useful to point out when gender dichotomies are false or exaggerated, but flatly stating that there can be no intellectual differences (or that all differences must be culturally conditioned) is dishonest. And it's a vulnerable viewpoint; if a true gap between the sexes is proven, does that mean we should go back in the kitchen after all?
It's a different matter entirely to ask whether men and women should be the same. Feminism can be prescriptive--instead of claiming to return to the natural order, we can fucking improve it! We've got technology now! And feminism is part of the technology that raises us above animals. Just as medicine frees us from our animal bodies' vulnerability to disease, feminism frees us from the biological destines of our animal bodies. The builds and "purposes" of male and female bodies may be hopelessly unequal, but as thinking humans we can repurpose those bodies to our own damn ends. A species that brought forth cities from the mud and fields from the desert would be pretty pitiful if it wasted the potential of half its population just because they're not built the same.
This silicon was meant to be sand but I'm using it to communicate with the world--my body was meant to be a baby factory but I'm using it to think and travel and save lives and rant on the Internet. It doesn't matter what I am; it matters what I can be.