I had three sisters growing up, so although I never actually saw them masturbating I heard enough squishety-squish behind bedroom and bathroom doors to know that girls whacked off. The search history on our family computer had enough stuff featuring naked guys, even gay porn for it to be clear they liked that too (our parents never even new how to turn the thing on).
When I was about 15 I was already six feet tall and had been working out for a couple of years with my buddy who got into boxing. Even my sisters expressed appreciation whenever I took off my shirt, which freaked me out a lot.
My uncle's wife, aunt Corinne, a small plump redhead with a wicked grin, who used to mother me as a little boy suddenly took an interest in me for a different reason. Her eyes, which used to look into mine with a caring expression, were now more likely to be scanning my body, widening periodocally, and all the time accompanied by that wicked grin. A lot of inappropriate touching went on as well, my ass was squeezed surruptitiously, and my abs rubbed; other muscles squeezed to check their hardness. All when no-one else was looking. This made me very uncomfortable and unhappy.
I knew two things, though. She wouldn't dare to make a full-on move on me (she was scared shitless of my Mom) and second, I hated her for being so creepy towards me. I eventually put the two things together one day and decided to clit-tease the bitch to death.
One day I had decided to spend some time in our pool when everyone else in the family was out, except for Corinne, as it turned out, who had come over to do the same. I put on a speedo that I'd grown out of some time ago and wandered outside where she was lounging in an a one-piece that hid her chubby 36 year old body. My package was barely covered by the material and my ass was hardly better covered. She almost fainted.
The next two hours I sauntered around in the little speedo, water dripping off me while she squirmed and surreptitiously squeezed and rubbed her crotch. The nasty grin slowly being replaced by a pained grimace. Eventually, she went inside.
I gave her a minute and then quietly put my ear to the door of the bathroom: The 'squishety-squish' I was used to seemed a lot louder than my sisters; she also had a really dirty mouth. Over the next two years or so I drove her nuts! I bet she blew a gallon of pussy juice around her pumping fingers thinking about my hard teen bod up till I went to college.