Is it immature/crazy to block an ex on instagram?

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We were in contact recently and even met up for coffee, but he failed to mention he had a new girlfriend, I found out from someone else. Our keeping in contact didn't mean anything but there was some flirting, I was just keeping in contact to see where it would go/what would happen. I didn't have any expectations, but obviously I wasn't expecting this. I backed off out of respect for his girlfriend, but didn't mention any of it to him I just stopped texting.

He recently liked a picture of mine on instagram (an acceptance letter to grad school). I think he meant this as a nice gesture, and I do appreciate that, but something about it bothered me. Am I crazy? Does it really matter that much?

I would still keep his phone number and everything... I wouldn't text him but he was my first real love and we have mutual friends and stuff so it would be weird to not have any source of contact with him at all... at the same time though I am not looking to get back together with him because I've seen his true colors. I don't know what to do...