It's not reductionist, Caryl, though it's a common misconception about radical feminism that it is in some way a form of biological determinism.
My position is this: I am smaller and weaker than most men. I am also vulnerable when pregnant and looking after a small child. That's biology. It should be irrelevant, because those facts in no way undermine my personhood, moral or political agency - that's feminism. But without acknowledging biology, it becomes impossible to name and explain where sex-based oppression comes from. Without acknowledging the reality of sex, sex-based oppression becomes as random as, say, deciding one day to pay blue eyed people 70% less, or denying them the vote, or denying them education, or cutting off bits of their genitals or forcing them to give birth if they get pregnant, or leaving them as the class of people overwhelmingly on the receiving end of sexual violence from the other class of people.
I happen to believe biology has to be acknowledged because of its role in explaining the oppression women historically and cross culturally are subjected to. To deny that it's important is to be like the sort of white person who blithely says "you know, I genuinely don't see the colour of people's skin..." It's an insult to the lived experience of people who find themselves treated differently because of their colour to pretend it's unimportant. Likewise biological sex. To deny the historical importance of biological sex in women's political and economic experiences of the world is to deny women the ability to talk about why they get a shit deal in a lot of cultures, a lot of the time.
I'm a feminist because I believe biological sex shouldn't matter when it comes to most of the important issues about being a human being.