I found out my bf had sex w/ another girl not long after we started going out. We weren't technically official at that point, but I really thought he cared and it feels like he cheated. What do I do?

It sounds like you're going through a really painful experience and I have to commend you for going the extra mile to gain outside perspective. You are a wise person for doing so! If you aren't already, ask for space to think and clarify your thoughts. It's easy to make decisions using our emotions, the hard part of this situation is to think seriously about all the reasons why you appreciate his presence in your life versus this action that he has done which seems to have caused some serious damage to the trust he earned from you. After calming down, tell him what you know without pointing your finger at him as that will cause him to disengage you will not be able to have a conversation. Tell him that this information makes you feel uncomfortable and also makes you unsure of the trust you have for him. Tell him that you appreciate and respect his honesty and will be open to hearing his testimony. Only then can you move forward with a choice that ultimately you must make yourself. I wish you the best of luck, always remember that your security and happiness lies within yourself. You cannot change the actions of another person, you can only control your own reaction. Be strong and wise, as you already are. You will find your way through this situation.