I'm thinking of being friends with a girl who I am interested in romantically but has a boyfriend
Why are you thinking about a girl who has a boyfriend?
She's a really beautiful girl and I want to have a relationship with her.
Trying to get with another guy's girlfriend is very disrespectful and cause him to be very upset with you.  If you want to avoid unnecessary drama and enemies in your life, you shouldn't ask out another guy's girlfriend.
I don't really care about other guys.
Okay, but she has a boyfriend and may not want to leave him. I would try and find someone who is single to pursue.