Girls don't mention their relationship status?

How come girls don't mention in passing whether they have boyfriends or not to me? Eg. I met a girl, I sat next to her, and another new male acquaintance of hers greeted her and said last time they met they had a discussion about her boyfriend. And I was like, "Oh wow she has a boyfriend." But then I'm curious why would a guy have a whole discussion about another girl's bf? Do girls steer their conversations towards their boyfriends when they meet guys they are not interested? That said, it's happening to me on a regular basis - am I just not discreet enough to make a point of finding out? Or is it a good sign that I'm not getting this "taken" vibe from them?
Girls don't mention their relationship status?
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