Most Helpful Guy
Kinda sounds like he was using you. Sorry.
Most Helpful Girl
I'd say... an excuse. You had sex with him before making it official, so in reality, he has an opening to leave without any strings attached. I may be wrong entirely, but if he is leaving you right after having sex with you, it may mean that he wasn't emotionally into you, but more physically into you. I have heard of people doing this, only in it for something from the other and leaving when they get it. It is possible that he realizes that you deserve someone, but only you know what went on, so I can only give my opinion on the information that you've given.