I think I can speak for all girls when I say that most of us have friends that are guys who we would never, ever, ever have any romantic interest in. Whether it be by choice or not, they have been friend-zoned so badly that you know nothing would ever happen. They are your good friend who happens to be a guy! Well guess what? Guys have girl friends just like we do, who are never leaving the friend zone no matter what.
When you're in a committed relationship, after a long enough period of time, your partner (hopefully) becomes your best friend. It's the two of you against the world through all the highs and lows. However, just because you are each other's best friend, that doesn't mean you can't have other friends, too, even if they're of the opposite sex. I think this is when girls get a little hypocritical because we get jealous and territorial. We mention our guy friends to our boyfriend every now and then asking, "Why did he do this? Will he like this outfit? How do I bring this up?" as well as other questions we can fearlessly ask. But when your boyfriend says he's going to hang out with this chick he's been friends with for years and you're like, "Uhm, no? Who is dis chick?" You need to put those insecurities aside and realize your man has friends that are girls other than you, and that is perfectly okay. You should be concerned if he doesn't!
If you are confident in your relationship and trust your partner, you should be okay with each other having friends of the opposite sex. It's healthy, socially normal, and needed to keep a balance in your life. I mean, don't be oblivious to obvious signs something else is going on, but don't become mopey and pouty if your guy wants to get lunch with a friend who happens to be a girl. Be confident in the foundation you have! You have your guy friends. So, let your boyfriend have his girl friends. I'm not saying you have to be totally elated about it, it's just something to be accepted. A little jealousy can be healthy, but not when it's unwarranted. Trying to isolate your man from women in general is just going to drive a resentful wedge between the two of you and chip away at your trust and confidence.
Your boyfriend loves you. You are his number one girl and you shouldn't question that. Having confidence in your relationship reaps so many benefits. Be confident and don't question your partner, let them have their girl friends! They'll appreciate you more and feel lucky to call you their own.