For me it was a former friends now wife, they met when we were at school and I know he took her virginity so she didn't have much experience with men. He developed a thing for taking her clothes off in front of me which evolved into her exposing her self to me regularly (which was before the Internet so the only way you saw tits or fanny's was in a magazine or real ones) and she was not shy about looking at my cock when ever she could. We saw each other naked a lot I remember having the duvet ripped off whilst getting a blow job and seeing the look on her face as she saw my erect cock for the first time, which when I looked at her I liked seeing her grinning and staring which turned me on more.
But the event that started it as thing, me, future hubby and her were in his parents caravan they had broken up but she was as before always around, he and I were sat opposite her I can't remember what was said but she just took her t shirt and bra off and stood topless she had amazing tits with big erect nipples, I remember her saying I'm not taking anything else off and sat down she sat with her eyes shut and we took it in turns to suck her nipples, then from nowhere she said I want to watch you wank, so I recall that doubt moment then thought fuck it, stripped off and sat in front of her completely naked stroking my cock looking at her tits for her to watch.
That event got her into exhibitionism more as it regularly became naked and open legged showing, several times she got him to fuck her in front of me and I can't count how many times I wanked In front of her after that.
And that one event lead to many good experiences showing and watching with several women who also enjoyed it.