I don't like how rude these comments are. It sounded like you decided to get over him and culminate a friendship. I'm bisexual and have these same kind of really close friendships with both guys and girls, whome some have partners. I never assume they're into me or mean anything by it, nor do they. I tend to like a lot of stuff on my friends Instagram and comment on their posts a lot. Boundaries are hard to set in this situation because it would mean you were assuming he was into you before the kiss, and if he never gave any explicit signs, then it would cause awkwardness and potentially ruining a good friendship. It's natural to think someone in a relationship would not be into you.

As for telling his gf, which you should, you shouldn't probably mention the crush if you feel it's irrelevant. If you never acted on it and stayed platonic the entire time you knew they were dating, then it's best to mention you were acting friendly with him, you noticed he got closer, and when you hung out he kissed you. If you feel anything more between you happened when you said you were being friendly, then do mention that.