I met up with an underage girl online and had sex with her.
Why would you have sex with an underage girl?
I don't know there is no specific reason behind, I just did.
It is illegal to have sex with a minor. This type of encounter could lead to some very serious consequences for you. It would be better for you to look for someone that is in your age group.
I don't care. She wanted to. I didn't pay for it.
Having sex with minors is illegal even if they want to participate. You really should just look for companions in your age group.