I flirted with a taken guy and now I feel guilty?

About a month ago I met this guy and I started liking him right away. He showed me a lot of kindness and he was being overly friendly with me. We work together so I was seeing him every day, and we became pretty close (at work). I started flirting with him a little bit and I think he flirted back, I mean he didn't stopped he, he reciprocated. It wasn't anything serious or big, but I was definitely flirting. Not even once did he mention that he has a long-term girlfriend! Not until another coworker asked him how's she doing in front of me. He hesitated to answer at first but then he said "she's fine but WE are not fine, haven't been fine for quite a time...". I was quite shocked and I said "I didn't know you had a girlfriend...", and he said "well, yeah...". My heart is a bit broken but I'll menage, the only thing that bothers me is this guilt I have. I mean to be honest I could've asked him! I could've asked someone else if he has a girlfriend, but I didn't. It never even crossed my mind to ask, I just assumed he's single since he never mentioned her. This situation really sucks, but the worst part is that I still like him... Is the blame on me? I'm not gonna flirt with him anymore, but the damage is already done, at least on my part.
I flirted with a taken guy and now I feel guilty?
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