Is AUTHOR wrong in the post ? POST : I know it sounds bad, please read the whole thing.

About six months ago a new coworker, Mike, asked me out on a date. I was upfront with him in saying that I wasn't looking to have a potentially serious or monogamous relationship. At the time, I had 2 FWBs and was exploring my sexuality. Mike told me he would ONLY want a serious and monogamous relationship, so I ultimately declined. He continuously brought it up over the week and I eventually broke down and just said yes to one dinner, "just to see if we were compatible".

Mike was (and is) a genuinely decent person and I had a good time at the restaurant. When he dropped me off at my apartment, he asked if we were dating. I was confused and figured he was asking if this was a first date, so I said yes.

I want to be clear that I think of dating as going out with someone to see whether you are compatible as a potential couple. I don't see it as being in a serious, official relationship unless it's otherwise stated.

I slept with one of my FWBs later that week.

Mike and I went out a few more times and I quit talking to my FWBs because I could see we were getting serious. He eventually asked me if he could "make it official on Facebook". I thought THIS was his way of asking if we could make our relationship official. I said yes and we dated for about 1 full month. It was after this point I realized he acted more like a 16 year old than a 26 year old. He also hid something from me that ruined any chance of continuing the relationship. 

We broke up, but we're still friendly whenever we have to interact with each other. One of our recent conversations was making fun of how short our relationship lasted. Mike said something about it lasting "almost 2 months" and I said, "Huh? It was only a month." After a little lighthearted argument, I realized he thought that when I said yes to "Are we dating?" he was asking if we were an official couple.

I told Mike what my definition of dating was, and that I didn't see us as an official couple at that time. His face changed completely and he asked if I had cheated on him between our versions of when the relationship started. I said no because I was ashamed. I felt very guilty afterwards, so shared this story with a friend and she said that it was crazy for him to assume anything without explicitly stating that we were official. Now I'm conflicted.

So AITA for cheating if I didnt realize that's what I was doing? Should I tell him the truth?

Please help.