Yeah sorry, I think expecting people to give up their seat because you feel you need it more is actually pretty unreasonable. You say they don't have manners, but here you are expecting people to do things for you for no other reason than you expect it. I'll give up my seat for someone I see as being in need (the elderly, someone with an injury, etc.), but I would never expect someone to move for me just because I'm an adult with children. I'm not disabled, I can stand just as well as anyone else. What makes you think that "every fit adult male with empty hands" owes you their seat?
IMHO ‘owes’ has nothing to with it. That implies paying a debt when I for one am talking about extending a courtesy. If to an individual this courtesy feels like something they owe then maybe the original purpose has been lost.