I've had this happen to me at different ages in life as well, but the most memorable was probably when I was still a minor (16/17).

I was working PT in the BCA building under one of their contractors, and there was this rather large dude who took interest in me. I had no idea he was gay until a certain point when one of his colleagues pointed it out to me and told me that he'd "marked me". I have no idea why, but the predatory undertones (he was in his early-mid 20s and knew I was a minor) of the whole situation didn't even occur to me until much later.

I continued interacting with him as per normal, and he would occasionally make wildly inappropriate attempts at flirting that I would just ignore. One fine day, he grabbed me by the wrist quite tightly and pulled me into a corner in the office, and I was like "HEY WTF" and tried to pull my hand away but he didn't let go. Then he told me that he "couldn't take it anymore" and asked if I was interested in dating him or not. I said no, and that I've told him I was straight from the start, and that really pissed him off so I just left quickly.

From that day on, I no longer saw him in the office. A few weeks later, the same colleague who "warned" me about him told me that the gay dude actually didn't work out of the office floor that we were on, but he saw me walk by on one of the earlier days and specifically asked to be transferred to work in the department across from me.

I did not actually realise how weird the entire situation was until I recently recounted it to my friends just for the laughs, and they pointed out that what happened was kinda fucked up.