Why would a guy lie about his relationship status to the new girl at work?

A new girl started at the job, and our male coworker was training her. They came into our office, and he introduced us. During our conversation, I asked about his girlfriend, and he mentioned that he was single. I apologized, as I assumed they were still together since he recently talked about her. I checked his Facebook, and even though he's active, both he and his “girlfriend” profiles still show them as "in a relationship with…” and full of pictures together. So, it seems like they’re still together online. I am under the impression that he’s lying about their relationship status. Why would a guy do this, IF this is the case?
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Why would a guy lie about his relationship status to the new girl at work?
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