The answer to your question is, "It's not okay." It's not fair. (Assuming you didn't do anything to alarm your brother about your interaction with guys.)

But the bizarre behavior of siblings can't always be reasoned away! Some people just seem to have this parental-like feeling of responsibility about younger kids in the family--including the way parents sometimes have a hard time with the transition from child to more sexual being. These protective older siblings may be mirroring their parents or someone else they admire; they may be compensating for parents who aren't involved enough; or it could be something else completely.

One thing's for sure: brothers can be extra protective of sisters--and sometimes that's thanks to old-fashioned ideas about women needing men's protection. Your brother may be trying to show off his manliness to his girlfriend or friends, whether it's on purpose or not. It could also be that his own opinions of guys isn't too high and he doesn't trust them with you. Maybe he knows lots of guys who take advantage of girls.

I'm not sure how helpful it would be to try and analyze your brother to his face. But if you feel like you can't ignore this issue, I would suggest maybe finding a quiet time and place to talk to him about how you feel. Is the problem mostly embarrassment? Or is the main thing that you want him to trust you? Whatever it is, telling him your actual feelings might help him see things from your perspective.

If you decide to have a conversation with him, it could also help build a foundation for a mutually respectful and supportive relationship in the future.