The guy I've been seeing slept with someone else last night, what do I do?

It's important to make known that we aren't officially in a relationship, we are definitely "seeing" each other, and have been since last November if everyone understands the difference. But last week he told me that he wanted to make things more serious, and even told me that he loved me. And now this. The girl was one of his close friends too, and I really don't know how to react. Do I have a right to be upset? Should I confront him, or not? Carry things on as though nothing has happened? Because we never agreed to be monogamous, but have been for months until this...