There is a line between casual relationship (what he probably wanted) and a full blown relationship (what you wanted and what happened)
The thing with JW guys though, is they have marriage marriage marriage drummed into their heads so much, that he probably doesn't know how to undertake a casual relationship.
So he ends up dating a good women like yourself and rationalizes with himself that it's OK because he would be thinking about marriage. Breaking some rules, but wouldn't get him kicked out of the religion.
Then the whole issue of sex comes up. He get's frustrated and inevitably (as the path of relationships go) you have sex.
That's when his guilt will kick in, and he'll just end up believing that this is a casual relationship, because in his mind, the only sex you have before marriage is "casual". But the kicker is, he doesn't want a casual relationship, he thinks he wants to get married. This is because in the JW religion, you don't date anyone unless you want to marry them.
So he breaks it off with you, probably because of guilt, and a lot of it is JW conditioning for him to think in a certain way.
The guilt will wear off, and he will (probably) do it again to another women, with the same end results as yourself.
And the whole cycle goes round and round and round....
That is a very basic explanation.