Once again, I'm on your side here, but IF you join with their assumption of "yes, a fetus is a human being that is afforded all standard rights upon conception", I don't see how it's a matter of semantics. If you and your friend were biologically connected, and you taking a certain pill would kill your friend, is it justifiable to say "well, it's my body so I can do whatever I want"?
The point is that their assumption is faulty in the first place, and that it *is* your body, and that you should have full control over that. But that argument, as you so succinctly put it, will not be effective for the above reason.