I agree completely with KingOfKong. Everyone has their panties in a bunch over stupid shit that he's not even positing. Wtf does misogyny or being a douchebag have anything to do with anything? Let me present you with two different situations:
1) A girl takes the bus to school every day. Every once in a while, she sees a guy from her physics class on the bus too. Sometimes, they talk about exams/assignments they had to do. Sometimes they make fun of the prof. Eventually, they add each other on facebook, and every once in a while, they talk about physics/professors/etc.
2) A girl takes the bus to school every day. One day, a guy she's never seen before sits down beside her and starts a random conversation. Before they say bye to each other, he asks for her number/email/facebook.
Without even bothering to dissect the minute details of the conversation, you have to have the social intelligence of a newt to not understand that the second guy is sexually aggressive and obviously interested. The way a guy's mind works though, the guy in the first scenario is probably interested too.
The reasoning behind what KingOfKing is saying is not just respect. Although I agree that respect plays a huge role in it, it has a lot to do with the possibility of developing romantic feelings. I'm someone who trusted a little too much in the past. I've been cheated on, because I used to think like a lot of ignorant and hopeful GAF members saying: "just trust her dude." Cheating happens. It happens all the time. It can happen with a friend. A random at a club/party. It could be one of YOUR friends (happened to me in high school

). Trust can be incredibly misguided. People with flirty personalities are people that tend to find themselves in situations where cheating is a possibility quite often. OPs girlfriend is clearly a flirty person. His mistrust shouldn't even stem from the fact that she's lying by omission (although in my books, that's a pretty big fucking deal), it should stem from his knowledge of the way his girlfriend is. She is flirty and impulsive. Terrible combination. But even if a girl doesn't have an inherently flirty personally, if she is constantly putting herself in situations where romance may develop with someone other than her boyfriend, the boyfriend has a right to be jealous.
Also, to the girl who was saying that she thinks her guy friends are hot but doesn't want to have sex with them: You are an idiot. Finding someone sexually attractive DOES mean that you want to have sex with them. That's exactly what being attracted to someone is. You think your boyfriend is cool because he doesn't care, but in actuality, he's a dumbass. That's not trust. That's just foolish ignorance. No matter how much you love your partner, if you keep putting yourself in situations where temptations arise, you are a terrible girlfriend. You could be the most faithful girl in the world, in theory, and still end up cheating on your boyfriend given the right circumstances are in place. And guess what? By constantly going out with men other than your boyfriend who you find very attractive is one of the circumstances. Add alcohol and maybe some dancing into the mix, and it's not a question of if, but a question of when. Go clubbing with attractive guys, have a little bit too much to drink, and guess what? A penis is inside you. And it's not your boyfriend's. Before my current relationship, I was that penis. I've helped quite a few girls cheat on their boyfriend's (I only found out afterwords). It fucking sucked to find out -- but I honestly wasn't surprised. A girl that goes to a party without her boyfriend, makes friends with a bunch of random guys and gets shitfaced is someone that has an incredibly high chance of cheating.
Is it okay for a girl to have have guy friends while in a relationship? Of course! Is it okay for a girl to put herself in situations where she might be tempted to cheat ("date" scenarios, 1 on 1 private hang outs, bar/club hopping without the boyfriend, etc.) while she has a boyfriend? In my opinion, abso-fucking-lutely not.