I'm attracted to a 15 year old girl
What do you think is going to happen with this?
I'm going to marry her and I'm going to take care of her as long as she lives
Not good.  It is wrong and illegal to be with anyone underage.  You can't move forward with any type of relationship with her.
No. I'm not doing anything wrong.
If you are an adult, then yes you are doing something wrong. You cannot be in a relationship with someone that is underage.