Well, you are a part of his life, and he at least wants you to be a part of that.
Really, this is a situation where you both need to meet each other halfway. He shouldn't be trying to make you go out all the time, and should understand that sometimes it's OK for him to hang out with his friends.
On the flip side, couples are expected to do social stuff, and presumably his friends like you or at least want to get to know you better. So, hammer out a compromise. If he's worth being with, he'll work with you.
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I agree that you both need to find a solution. Schedule a time when he and you can hang with friends and a time for just two of you. Make him know you care, but also that you need time to just be you.
Im an introvert dating an extrovert. He is very out there and does not shy away from activities or opinions. Whereas, I am shy and much more withdrawn. The dynamic here is that we give each other what we cant get from others. Another introvert would not lead the relationship or provide me with entertainment the way he does, but in turn, I provide him calmness and attention alone that he doesnt get from an extroverted woman. Its interesting, and it can work. But as it is, there are other problems too =/.... He feels that I am boring sometimes while I feel that he is too pretentious and fake at times, leading us to be ambivalent about one another. Who knows where this will go.....
I am an extrovert and my bf has used your exact words. Negotiate. Find time and space for both of your needs.