I have found that even when two people agree to be "completely honest" about everything, that still often means something different to each of them.
I think that he probably thought that telling you might've hurt your feelings, and so his first inclination is to protect you from hurting, coupled with protecting himself from any possible fallout. Often I find that men just cannot believe that women want the whole story - they've been programmed to keep us safe, or what they perceive as safe. Hence, we are in the dark a lot of the time.
I am curious, though. If it was an innocent situation in which they just bumped into each other while out and about, why was there a text message? Who followed up the encounter with a text and who responded? That seems like going against your ground rules, too.