Two comments from me on this one.
First I am not going to introduce sex into a relationship until it has gone to being an exclusive and committed relationship. This is certainly just me and I would suspect that this is not going to be the norm for a lot of guys.
In your particular situation you indicate that you started out "fast" and that YOU were trying to slow things down. I would suspect that your guy does not have the same values that I have and that he had not gotten into your pants by the whatever date that he lost interest. I would also suspect that if he had gotten into your pants by the whatever date that he would have accomplished what he was after and would have also not been interested in continuing with you as he would now have his sights set on his next conquest.
I do that too- I generally wait on sex until things have gone past infatuation and further into serious coupling. *grin* Kissing, snuggling, making out- these things I just go with the flow. But I put the brakes on sex until I know more of what kind of person they are.
To the OP- from the information you've given, it sounds like this is a sex question- but keep in mind that may not be the case. Minority opinion, but there are other early relationship killers out there. But basically I'd say he discovered some incompatibility that finally made him give it up. Might have been sex, who knows? Like these other fine people have said- better to know now than get burned later. Good luck in your future endeavours, lass! There's bound to be one out there that makes you feel just right, who feels just the same about you. Who knows? Maybe he'll be the one for whom you don't need the rules anymore... *grin*