I find my friend's girlfriend attractive and I'm going to stare at her ass.
What would motivate you to stare at her?
I want her to be mine, but she is taken already.
That is an impolite and rude thing to do. It's wrong to sexually objectify people. You only like this girl because of the way she looks which isn't fair to her.
It's not rude at all. Her boyfriend doesn't care.
Whether her boyfriend cares or not, treating this woman as nothing more than a sex object is immoral and wrong. Have you considered her feelings? You doing this would surely make her feel uncomfortable and uneasy.