I really want to cum on my girlfriend without asking for consent.
I hope you are kidding because why would you cum on her without consent?
I love to see her face of surprise when I cum on her face without her noticing.
That is really wrong. You should ask for consent before you do something sexual to your partner. To do otherwise will drive them away, and could get you in trouble.
Why does it matter to people? I think it's funny.
It matters because it's a form of respect and care. So I am pretty sure most girls would be okay with it if you asked, so why take the risk of an argument? You will be happier and enjoy the sexual experience more when both parties consent.