Is it inappropriate for a guy with a girlfriend to go to dinner with a female coworker after work?

Yesterday night after work, my male coworker asked me if I wanted to go get some dinner. We split the bill and nothing romantic went on. Is this a bad idea? I don't want to hurt his girlfriend. I have no idea if this guy likes me- he has never explicitely put me in the friend zone or shown romantic interest.
Is it inappropriate for a guy with a girlfriend to go to dinner with a female coworker after work?
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