If a fish were living inside your body, you'd have the right to expel it, too. That has nothing to do with the treatment fish deserve or don't deserve, and everything to do with human bodily autonomy.
Expel a human even if it kills the human? That's a tougher sell. And that's always been the problem. Yell as much as you want about "a woman's right to her own body", but there's something there besides her body, something that is 1) still genetically human, 2) genetically different from the mother, and 3) will die if expelled by the methods used. It's not just the woman's body.
I think of it in terms of the Declaration of Independence: "life, liberty, and the pursuit of happiness". Well here, "life" is at war with "liberty and the pursuit of happiness" in a way where, whichever one wins, the other has to lose.
EDIT: What part of this is getting downvotes? I'm honestly curious for a distinction here.