Kept in secret...

So, here's the thing. Seeing someone and we're exclusive. Nothing official yet though. However, I feel very much connected to him. I really like him. What I dont like is the fact that he keeps us a secret. Like, when we are together and somebody calls him, he lies and tells them that he's somewhere else, rather than with me. I feel like a mistress, being kept like that. What's up with that?