That being said, after thinking about it, I think I may agree with you on this (even though I am not a BY liberal). It does seem a bit strange to constantly address high school girls as “future wives and mothers”.
First of all, they should be focused on what you are now. Second of all, our main tafkid is being Ovdei Hashem, and being a wife and mother is merely the way we fulfill that role.
A single girl I know was once asked to speak on the topic “The tafkid of a single girl”. She refused to do it since she thought it was ridiculous. She was like, “What do you mean ‘the tafkid of a single girl?’ It’s the same tafkid as any Jew’s – being an Eved Hashem.”