Help Please

The other day i found out my SO was looking thru Casual Encounters on Craigslist and it was more than once (i checked his history on PC) i have came to the conclusion it looks like he is actively seeking to cheat. However, he said he was just curios to see what ppl put up on there. But my trust for him took a blow a few weeks ago that now i just dont know.

A few weeks ago i found out he had girls numbers on his cell phone but had switch their names to guys (about 6 girls) to keep me from finding out! After hours of debying he finally admitted he did it to no hurt me. That what if i let him, those ppl were like his back up? Put urself in my shoes.....What would you do?? Leave your suggestion below and thank you.