Gem, friends and long term relationship

In the course of about 8 years, we started as good friend, we became best friend and we've been dating for 3 years now. We're still best friend but we changed to love each other deeply. 50% couple, 50% best friend. Hard to describe, most of our friend don't understand our relationship but we do 🙂
My boyfriend is very friendly with the women in his life. I don't mind it one bit. His feminine side is very strong. I was told it's a Gemini trait. We go shopping and go to Starbucks together, and we will talk about what we both like which is pretty much everything. We love the same things. It's amazing 🙂

There's something that bugs me and I want to know if it's normal for Gemini men.
He has a female friend that don't know I exist.

My boyfriend graduated college recently in a field with little to no female in his program. Mechanic engineering. In the last year, he's met maybe 5 females in his classes. In a program with 80 men.
Being so friendly with women, getting along with them more than with men, he stuck with them.
After graduating, he exchanged numbers and added some of FB. We are FB official, so they know he's taken.

My boyfriend text a lot and I never ask with whom. It's his life. I'm not jealous, he is however very possessive of me. But I know, or so I thought, all his friends.

Last week I was looking at his Snapchat best friends for fun and saw that I had lost the first spot to a girl called Karen. I didn't know any Karen in his life, so I asked about her without real intention. Just out of curiosity. He told me she's a friend from school. I mentioned something about him never mentioning her ever. We started talking about friends, and how he doesn't think it's necessary for me to know all his friends. To me, it was a shock as I always thought it was just common sense for couples to know each others friends and such. He also said something about not giving a fuck about who I'm friends with. Again, another shock, as we talk about our lives all the time. They apparently snapchat a lot. Using it like texting. He text her a lot too. I don't know about what. When I said something about inviting her over like we do some friends, he said that she doesn't know I exist because he never mentioned me to her. She's not on FB, so I'm pretty sure she doesn't know he's in a relationship. He assured me she's just a friend and nothing more.

Gemini likes to have secrets, but we're in a serious relationship. How should I approach it to him that i