I was in a 2 year relationship with my boyfriend when this happened. It was a great relationship, and I really loved him. I honestly thought he might be 'the one', and not once have I ever thought about the possibility of cheating on him.
I was regularly going to a massage parlor once a week, and that was going on for years. My masseuse was a girl, she was actually a good friend of mine, and I've never had a massage by someone other than her. It was supposed to be the same that day, but when I came to a parlor and prepared for a massage, she told me that she had something urgent to do, and that she had a replacement for me if that's ok. That was a little weird since she would always tell me upfront if she couldn't massage me for any reason and we would just cancel it, but it was still ok for me.
The replacement masseur was a man, which was again a surprise for me, but I thought nothing of it at that time. I also didn't think anything about his looks, I basically didn't notice him, only later I would realise how hot he actually was.
I lied on the massage table, only in my panties, as always. In my regular massages, I was never wearing a bra (as it was in a way for a back massage), but my masseuse would always put a towel over my boobs when I was lying on my back. So I did the same as usual, not really thinking about it too much.
His massage was very nice, and I felt completely normal at the beginning. I did, however, realise after a while that it's in fact a little weird that a guy is massaging me, especially while his hands were around my upper thighs. That was the part where I thought that this could be considered a bit weird and sexual, but I wasn't aroused at all at that point, it just crossed my mind.
And for about 20 minutes, while he was massaging my back and legs, he was completely professional.
I guess the first sign of something unusual was the fact that he didn't put any towel over my boobs once I flipped on the back. Instead, he just started to massage my neck and upper chest, with my boobies completely on display. And it was just too awkward for me to say anything about it. "It doesn't matter" - I was thinking to myself. I would often go back to that moment in my memories, thinking that maybe nothing would be the same if I had just asked him to put a towel on. It would probably be a clear sign to him not to try anything.