For me, it started during an affair that I had. (Yes, I know. gross, I was only 19). It wasn't the goal of that exact night, but I ended up liking it quite a bit. So, I suppose it was while dating? But I picked while dating and other to kind of cover all the bases.
However, after that, I only let one other man try it with me because I trusted him and had known him for so long and had slept with him off and on through the years. No one else has done it since, and it's been maybe 4-5 years, but I wouldn't mind trying it with my soon-to-be husband.