I Have a Question

Many reasons, most of them mentioned here but will agree, maybe wasn't as good or what he expected, maybe he's a selfish douche, maybe he was cheating, definitely immaturity and just wanting to get laid. Curious when you say begins to ignore, do you mean immediately after (sometimes that's sexual guilt) or you mean for days after (just wanted sex). Truth will come out if he contacts you again and what the extent of the conversation is. If you feel it is going out and leading to sex again you have your answer. Best way to answer it is tell him you want to hold off on sex for a few weeks and see if he sticks around.