Because I have always worked around men, mostly young men, mostly men good with computers, often single, and don't believe in relationships with coworkers, it's a line for me.
It freaked me out when a coworker gave me a hug in the parking lot after listening to him vent about stuff. We were off the clock but still on work property -- I asked him not to do that because I didn't want people to get the wrong idea about our friendship.
I also don't want to be propositioned, jokingly or not. I'll usually start showing disinterest with an emulation of Daria getting aroused -- deadpan, totally toneless "Oh, baby. Oh, baby. Oh." Which takes care of the joking comments with a joke back, but clearly shows I'm not impressed.
The one time unwanted attention continued, it was because we were all stuck in a van together to go to a training site. I'd already indicated I was taken, but he wasn't paying attention. He also wanted to torture a young sheltered kid on the rollout -- a homeschooled Mormon. So when we were making good time and there was a sign for a XXX bookstore, he wanted to stop and the rest of the people aside from myself and Mormon Boy said they wanted to as well. The driver stopped.
I could have sat in the car. Could have contacted my supervisor who I was trying to impress with my independence and ability to handle anything. But decided instead to fix the root problem -- these guys obviously perceived me as fun to tease. I also didn't want the Mormon to be the one embarrassed in the store, and they'd have tortured him if he didn't go, and if I didn't go he would be the only target in there.
So I went in, went straight up to the BDSM section, and asked the store associate if I could take a look at their cat-o-nines, talking about this lovely buttery-soft one I'd used before and never been able to find the like (truth, heh). The store associate offered himself for demonstration purposes, though when he started to take off his shirt I said "Nah, it's okay, I'm more wanting to feel the weight, not see the stripes." (Wasn't going to encourage nudity.)
Then said none of them were really what I was looking for... but saw a little toy version, obviously not a real whip... "But I can't leave without buying something."
Bought the toy-sized cat-o--nine, clipped it to my keychain, and then acted like I hadn't thought all this would have created an audience... turned around, saw everyone watching, said, "Oh, you guys ready to go?"
They were. I kept the toy whip attached to my keychain until that guy departed for his route after training, which he seemed to take as a hint. 
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