This happened. If you find reluctance off-putting, you may want to pick a different story.
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My first job out of college was at a small technology company. I was 22 and had a steady girlfriend. I quickly became friends with a girl in the office who was just a year older than I was and who was already married.
Kristen, my new work friend, was cute, but a little bit on the plain side, and not someone I'd ordinarily notice. I was happy with my girlfriend, and Kristen loved to go on and on about how amazing her husband was. She seemed to enjoy "one-upping" everyone in the office when it came to her relationship, and how it was so much better than everyone else's.
I'm sure you know the type.
We quickly settled into a kind of brother / sister dynamic, where we'd make fun of one another all the time. While she wasn't my type, I'd still find myself checking out her body regularly, as she was petite and curvy at 5'5", 115 lbs, and had full, C cup boobs.
Within a year I'd started working on a project on the other side of the country, and while Kristen and I would still instant message one another, we weren't as close as we'd been. My girlfriend and I broke up around this time, and I soon left the company altogether.
One day, months later, Kristen emailed me confiding that she'd found out her husband was sleeping with another woman, whom he'd gotten pregnant. She was as devastated as one would expect, and I genuinely felt sorry for her as she went through the divorce process. After a few months she was ready to start dating again, and I set her up with a good friend of mine, thinking they'd be compatible.
Kristen and my friend dated for a couple of months before breaking up. I'd found myself missing my friends back home on the east coast, and moved back after a three year stint out west.
I was grabbing drinks one night with the friend I'd set up with Kristen, and was curious about why things had fizzled out between them. I really didn't think of her romantically, which is why I'd set them up in the first place, but what he told me began to change my perception of her.
Kristen and I would see one another from time to time among other friends, or go out for drinks where she'd be my 'wing woman' and / or I'd be her wingman. It felt very easy and platonic, but I couldn't get what my friend and said about her out of my mind.
She and I were both obsessed with a particular TV show, and had talked about watching it together. I invited her over to my apartment one night to watch the show. She was happy to come over, and I'm sure she didn't think twice about it being anything like a date. I'm still not sure how I was viewing it.
We were sitting on opposite ends of my couch drinking wine and catching up while we waited for the show to come on. I still hadn't gotten her side of the story as far as why things hadn't worked out with my friend, and I had a smile on my face as I started to ask her about it. She gave me the standard, anodyne "Oh, you know, sometimes things just don't work out..." answer but as she noticed my expression she smiled and said "Why? Did you talk to him about it? What did he say?"
I liked playfully mocking her in general, but it felt particularly fun to feel like I "had" something on her. And the wine helped me make the decision to test out a new way of teasing her.
"He said that neither of you was really ready for a relationship, but he also cryptically described you as being 'a lot of fun.' When I pressed him on it, he said that he's never been with someone as sexual as you are, and that you seemed like you were constantly in heat."