Is it OK checking phone of ur partner?

I was with a guy for 2yrs. Never checked his phone. My phone got dead one day.i asked his to see maps and a text popped up Opening his whatsapp I saw atleast 4 girls with whom he has been having sex chats for a long time. Then some on instagram. Sex chats with some random guys too.

I just didn't know how relationship work. This was my 1st. I just thought if I am doing so much for him(I take his parents for any health checkup etc as he is doing his studies in anotherstate),he would be loyal & i don'thave to check his socialmedia. Guess i was a completefool .