Interesting post ss86. Thanks for sharing. It's a shame he is already taken; sounds like you really liked him. You asked if he is being out of order to his gf. Not at all. Men can talk to whoever they please, in and out of a relationship. Same for women. He isn't flirting with you or inviting you on dates so I see nothing awry. If he does start flirting or being inappropriate, you can simply ignore him and he'll give up. No worries.

As far as responding to your social media, perhaps he finds you to be an interesting person and likes reading your posts (men sometimes get presented as impossibly sexual about everything but that's just a reductive stereotype) or perhaps he was flattered by your compliment to him and enjoys attention from you. The latter situation could turn into something you don't want but you don't yet have enough info to tell.

Either way, I think the important part is how you feel. Are you comfortable messaging with this guy knowing that he won't date you or would you rather not communicate with him at all? That's entirely up to you. Do whatever feels more comfortable and peaceful for you in your own life. I would discourage you from trying to guess his motives or thought process. I don't see any value in that for you.

If you are interested in finding a partner, I hope a lovely single guy is in your future! No reason why he wouldn't be! And fair play to you for being brave and telling the first guy how you felt before you knew he had a gf....nothing ventured, nothing gained, right? That courage could serve you well in the future when you meet someone interesting who is also available