do girls with boyfriends feel annoyed when other guys are interested?

[stayin alive]

I striked up a friendly conversation with this cute girl in class and she seemed polite. But later when I asked for her number she suggested exchanging emails first but after a pause said it's OK if she gives me her number so I sensed that she wasn't comfortable with it. At the end of the day I say see ya later! but she sorta frowned and didn't smile or anything.
Later I saw her facebook pic and it shows her close to another guy so that explains why she wasn't as friendly.
Do girls in relationships feel annoyed when guys are interested? Or do they just never say it and wouldn't mind making a new friend?

[Chris]

She might be one of those girls who thinks any guy being nice just wants to get in her pants.

[On Your Knees]

I'm not a woman, but I know some who are in relationships with guys and still go on dates with guys other than their boyfriends, obviously everyone is different though, some might get annoyed if a guy comes on to her that isn't her boyfriend, other women might be flattered by it!