Mrs. E married early and was a couple of years into her divorce. She'd only been with a few men before that and hadn't really had a ton of "grownup" single life experience. She was dating but disappointed that she was sort of looking to revisit some of what she missed, and most men she was meeting were either trying to marry or remarry. One brought his bank statements to a second date thinking that would impress her. So she had an on-and-off boyfriend she was seeing, and dates here and there, but was generally disappointed. She was raised with a fairly traditional background and actual casual sex without the formalities of dating struck her as pretty taboo.
I was single, dating casually, and - relative to her, though not radically so - the more experienced one. She knew that I usually had multiple women going and was interested in spending time together but thought it was too wild for her, so we were just coffee friends for a while. We'd talk openly about our dating lives and flirt a little.
The tipping point was a very frank conversation on a Sunday morning at the coffee shop, where she admitted she was having a little identity crisis because she'd woken up with her pseudo-boyfriend Saturday morning and went on a date that night that ended in unexpected oral sex. She was scandalized by it and kind of fixating.
I politely explained that that was really nothing, and that that's what it's like sometimes if she's going to actively date. Admitted that I'd once worked through three women in my rotation in one day - woke up with one who knew I had a lunch date and went for it anyway, brought the lunch date back to my place, then had a dinner date at the third's place - and that hers was really just not a particularly shocking story.
Something clicked for her there that I really wasn't judging her and that she wasn't expected to take herself so seriously. We ended up going out for some live music that night, debated what to do next in the car, and she spent the night at my place, we screwed twice. Kept going after that while continuing with our other activities.
Never had some big "what are we doing" conversation. We just both liked spending a lot of time together and kind of tapered off of other partners over about 18 months. Had some adventures in there but never really thought of ourselves as "non-monogamous" as much as just single together, until we weren't.
I'd wager there's probably a small generational/regional cultural gap, here, too. Some people on this board are in an age group where it was taken for granted that being single implied doing whatever you wanted until you found a new relationship, and that most women have someone to call. Others took it for granted that being single meant you probably weren't getting laid regularly, or at least either fell for it or cooperated in playing make-believe to be polite.
Notably, the poll results are currently pretty divided between "neither of us" and "both of us". I'll bet it'd correlate with some other demographics.