well..it depends on how he was looking..a little look..sure..that can be flattering..if I like the guy. If its just some random guy it might be weird or even rude. Trust me. Some guys can give a look that is flattering..and others you feel like you were just raped!
I'm bad at catching stuff like that. I was at a department store with my mom a week ago and the guy you show your items to before going into the dressing room (so you don't steal stuff) was creeping. Then when I was walking in, there happened to be a mirror right in front of me so I saw him staring at my butt in great detail.-.- It's not a turn on to me. It just makes it awkward to walk by again.
This has happened to me recently. I was talking to a guy at work and turned to walk away and realized I had to tell him something else. When I turned back, he was checking my butt out. I was totally flattered. One reason is that he isn't one of the nasty old men that I have to deal with sometimes. He just happens to be one of the sexiest guys I work with.