I slept with a guy while I was unofficially seeing someone else. Was this wrong?

No. You were not in an exclusive relationship, or really any kind of relationship. I don't really know your story and why you have to be together only secretly, but this was not anything committed. If he's upset, it's his problem. He should have taken what steps he needed to take so the two of you could be together officially. Unofficially has no obligations. Every guy's dream situation is to have no strings attached sex with a girl and have her be devoted to him while he does whatever he wants. That's not the way the world works. The sooner he finds that out, the better. Maybe you taught him a lesson.