I'm no relationship expert, but I think it can mean 1 of 2 things:
1. He really likes you enough that maybe he didn't fool around w/ anyone else because he wants to take your relationship 'to the next level' (and by telling you what he told you it was your little clue)
=OR=
2. He actually did fool around with girls there, but put up the defensive and told you he didn't just in case you cared.
*hehehe* OK, so I'm not professional, but that's what I think
Boys are weird and mysterious.. I don't think we'll ever figure them all out. Have you noticed that for every ONE relationship question on these boards that come from a guy asking about a girl, there are 500 questions on the boards coming from a girl asking about guys?
See? We'll never figure 'em out