What do you think of me asking for more money after an agreement has been reached?
That seems greedy. How can you ask for more money after reaching an agreement?
I'm a man and I can make demands.
It's unethical not to stick to the agreement and demand more money. Nobody will trust you if you act like you're entitled for more.
Well I can just keep getting away with it.
You might this time but it will hurt your future income to break a contract. Respecting a binding contract is the least you can do to gain a person's trust. And by breaking it, it means your word means nothing.