trust issues with my boyfriend

My boyfriend and I have been dating for about a year and a half. Coming into this relationship, he was coming out of a 2 year one. So at first, when he was messaging/sexting his ex I understood it was probably cause he wasn't over her. But then six months in, he made plans with her again. He has never physically cheated on me, but I have found messages with other girls a few other times and I grabbed his phone the other day and her instagram profile was pulled up.... should I assume the worse or just let it go? Are guys just guys or do I have a right to be furious?