Why would a guy brag about a one-night-stand?

Because he has an underdeveloped since of himself and is not happy with the fact that he's alone and should be committed. While I do not condone cheating if he really wanted to keep it a secret he wouldn't have told anyone. He was honest enough with you to tell you he was in a relationship so he should be man enough to keep his private life private. This was his way of beating on his chest around a group of guys, to be honest if he was really in love your one night stand would have never happened. Nothing against you at all but if he's that damn bold he's just a dog looking for a leg to hump and he found yours. I would suggest you avoid him in the future this way you won't get caught up in any of his future drama. You were both consenting adults that should have been it. That he has to brag about it with his "boys" says that he has no class and you don't want to get caught up in any of his future drama.

Because convincing a girl to have a one night stand with you is HARD WORK. As a female yourself you might not understand this...it doesn't mean that he's an asshole (though he might be) he's probably just proud of his accomplishment or still in awe regarding his luck. Bringing it up not only lets his friends know it happened but confirms that such things really are possible--he might start wondering if he imagined the whole thing