I want to look at a girls ass
What is causing you to be so interested in looking at her ass?
Because it's hard to see when she wears loose pants.
Glancing at someone from the corner of your eye is one thing. But, to stare at someone's butt is not only rude, but also creepy and could very likely make the girl feel unsafe and uncomfortable. Be respectful of the people around you!
I'm looking and it's not my fault if she wears loose pants.
Staring at a girl's private parts will not only show you that you are disrespecting her, but may also cause other people to get upset. Just think that you should keep your stare to only a glance if you truly want to look.