Guy's Behavior

Is it normal for guys to hide that they're in a relationship?

So there's this guy I've been working closely with for about a year now. We're pretty good friends and he's been showing hints that he may like me. Never once did he ever mention he was in a relationship.

Well, one of my other coworkers was being nice and informed him that I liked him. I guess he got really defensive and said he was already in a relationship and when my coworker asked, out of curiosity (since no one knew he wasn't single, not even the other guys who always talk about their girlfriend's), how long he's been with them he said he "doesn't mix his work life with his personal life". He's always seemed socially awkward so I don't know if maybe it's a new relationship and he's unsure or what it is.

He ended up texting me immediately stating he had no idea I had those feelings for him and that he's currently "in a relationship at the moment" and that I'm a really cool person. Which was nice of him, we're still good friends and all is good but I'm curious...

Is that normal?
Is it normal for guys to hide that they're in a relationship?
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