My girlfriend always talks to other guys but it's not like she's cheating on me or anything because I've read her text message conversations before but as a guy I know I'm not really suppose to talk to other girls while in a relationship so I don't really understand why girls are allowed to.
She is a "Tester". This type of woman keeps a group of about 5 or 10 guys lined up at all times, comparing them among one another. She probably has as many or more guy friends than girl friends. Take a count whenever you can. If that is the case, she's a "Tester".
You better be your best because you are in a competition, and when she finds someone she thinks is better, you're in trouble.
Now the theory is you need to break her and convince her you're the "one" before someone else does.
You're allowed to talk to girls while being in a relationship. A lot of my male friends are in a relationship and they still talk to me. I talk to them because they're my friends and I'm also in a relationship. 90% of the people I talk to are males. Why should a relationship change who my friends are?
Being a relationship doesn't stop anyone from making friends or talking to others. Both of you have the right to talk to the opposite gender but if she's controlling you when she herself is talking to other guys, it's time to have a talk about this.
Just because someone is in a relationship, it doesn't mean members of the opposite sex suddenly disappear. They're still going to be friends with them, they're still going to have conversations with them.
There is no universal standard rule for this situation. People of the opposite sex can be just friends without any romantic or sexual feelings between them and there is no need to avoid friends simply because you are in a relationship. However, you may need to do that if you are in a relationship with someone who is jealous and not trusting. If that is the situation, you may want to reconsider the value of the relationship that you are in.
What are they supposed to do? Sit in their house 24/7?
Who said she's not supposed to talk to other guys & you're not supposed to talk to other girls? Maybe those guys are her friends. Maybe those guys have some project together at school/university. It could be anything. Don't get suspicions too easily.