I actually had a similar thing happen to me, but it was on fabebook. I dated a guy for a year, we were both in a monogamous relationship, but he refused to respond to my facebook friend request, keeping me hidden from his page the entire time. His list of friends were family members but also very attractive women/models/dancers he barely knew from his work as a musician. I would bring it up about how this made me feel, and if he loved me and I was that important to him, why wasn’t I on there, but he would tell me FB didn’t matter and I how much he hated social media and what kind of trouble it caused. He was very private and didn’t like his personal life on there. Long story short, he was cheating on me and being inappropriate. I know this because we have talked about what happened back then as we are friends now, 2 years later.He knew he was wrong and said he just liked the attention he got from other women which led to him cheating. Hope that’s not the case for you. Good luck