Does he have the right to be mad or not .

So my boyfriend, well my ex-boyfriend now gave me his old cellphone to use because I don’t have a phone at the moment. However, he didn’t delete his messages on the phone so curiosity got the best of me and I read the text messages. From the messages I found out that he is fucking three other women I don’t know about but whenever I have asked him before whether he is seeing someone he claim not to. So I confronted him about my findings but instead he turn it around and said I invaded his privacy by reading his messages. So ladies does he have a right to be mad? Did I violate his privacy? If he didn’t want me to see the messages he could have deleted them before handing me the phone