Thank you for reminding.
So in essence, only male LPRs between 18 and 25 may be required to go to US military... When it comes to US citizenship, I think the circle is much larger for those who'd be serving the US when required.
Theoretically, if I understand correctly, under current law LPRs cannot be forced to dig threnches and do something else for defense reasons. But citizens of all genders may be required to. Hence it is part of oath...