Or not.
Some women advertise the ‘fact’ that they have a bf to show that they are not losers who can’t get a man, thus, insuring their market value. Even some hot women do this, to let the new guy know that they are hot properties. If the man gets over that hurdle, the woman sometimes will then say “Well, we’re not that serious” and is actually open to a date/relationship. Not totally uncommon.
Unfortunately, in a sales situation, it’s extremely difficult for a male to discern what the true intent is.
You may think it’s not obvious, but slobbering isn’t actually subtle, you know. 
As a zillion people have pointed out, you took what everyone else is saying is the least likely outcome and running with it. Of course you’re going to get pushback.
Funny, I was just thinking about this thread today. I was at Subway and the not unattractive Western woman ahead of me in line was also attempting to order in English. When she looked my way, we made eye contact so we chatted briefly, as fellow Westerners often do in Taiwan.
When I asked what she was doing here, she replied "We’re here for work, I work at the AIT (which functions as the embassy).
Did she say “we” because:
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Some people say things without it meaning too much.
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To gently head off any pickup attempt.
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To keep herself from attacking me then and there in the store?
She was telling you she is there with a significant other.
You should have asked her if this guy was real or not.
Otherwise, you are as guilty of being a creepy rapist as the OP.
IOW, no one knows jack shit and it’s all people projecting their past experience onto the OP.
An utter shit show, quite frankly.
I think one thing that hasn’t been touched on is that men can do it too, and it’s often done in exchange or response, as a human social code. I give the cashier a smile and we chat a bit, she happens to mention at she has a boyfriend, so as to politely forestall me trying to get her phone number, which probably happens a dozen ea day. No big deal, I counter by dropping a mention of my wife, and we’ve satisfied ourselves that the code’s been observed and nobody’s made an unwelcome move, embarrassed themselves by misreading signals, or had to make an awkward rejection. We might even be safe for a bit of harmless flirting, now that we both know that there are no real stakes on the table. Human codes are funny in just how much they can convey in a couple of words.