He’s cheating but what is he thinking?

There’s a guy I’ve been sleeping with. He’s got a girlfriend (I know it’s bad, the question isn’t on morals and stuff). We’ve known each other years through work, we always had banter and a flirt like friendship. He kissed me once and it has gone from there. He asked to kiss me. We slept together and I thought it may just be sex and that’s it but it’s continued for 6/7 months. We work together and he always seems to want attention (we both do) but we make out a lot at work on our breaks and stuff. But I don’t know what this is. It’s not just a sex and leave me it is full on and continuous. Could he have feelings? Why wouldn’t he have stopped by now? He’s also started doing things like flirting more openly at work. Had to sign a birthday card and he said put from me ans you. So a colleague commented that it would look like we were a couple. And when I joked about it he said well we already do because our job has always coincidentally made us move locations together. I just don’t know what this behaviour means
He’s cheating but what is he thinking?
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