Sister and friend.
Even now I still can't believe I actually did this, let alone that I'm writing about it now. For my sister's 14th birthday she took about 10 of her friends to the local indoor waterpark and, being at a loose end, I decided to tag along. I was sixteen at the time and I was starting to notice just how sexy my sister and some of her friends were. There weren't any male/female changing areas as such, just a lot of small changing cubicles that could fit two people (or three at a pinch). Seeing that it was my sister that was paying, it ended up that she, her best friend and me were the last to get to the changing area and could only find one free cubicle. After arguing for a minute or two about who should use it first, I finally suggested that we all just go in and keep our backs turned.
Both girls got as far as their underwear and then (my sister glaring at me threateningly) told me to keep facing the other way. I managed to sneak a quick look, but to my disappointment they'd both put their towels around them whilst they slipped off their underclothes and wriggled into their swimsuits.
As I began to undress I started feeling more and more horny, liking the idea that they might be (knowing them: were) sneaking glances at me. By the time I was down to my boxer shorts, being naked in front of these two hotties was starting to seem like a very good idea and I fought to keep my erection at half mast so that they wouldn't guess how much I was enjoying what came next. I slipped my shorts down my legs, my heart beating wildly as I felt both their eyes on my now naked back. Taking my time, I searched through my bag for my swimming shorts and then, having found them, turned until I was almost side on to both girls and pulled them up. As we exited the cubicle I acted like nothing had happened and quickly headed for the water whilst the girls held back, whispering amongst themselves.
After about an hour and a half had gone by everyone had had enough and we decided to call it a day and go get something to eat. I'd almost put out of my head what had happened when we'd first arrived, but when I got to the changing area my sister and her friend were there beckoning me into their cubicle despite the fact that there were a few others available.
The girls disappointed me a second time by using the towel trick, but I knew now that I could enjoy the experience just as much by flashing them again. This time I was naked quicker as I only had to take off my shorts and I kept it that way longer as I slowly dried myself with my towel. After a while I was starting to get hard again, so I reluctantly slipped on my shorts and pants, but not before once again giving the girls a brief but good angle to see my cock.
I wasn't that hungry, but was now desperate to get home and jerk off over what I'd done, so I took off for home leaving my sister and her friends to get some fast food. When I got home I was glad to find nobody in, allowing me to strip off and take my time slowly massaging my dick whilst replaying the days events over in my mind. After a mind blowing orgasm I decided to take a bath to get rid of the smell of chlorine and wallowed there for about an hour.
As I stepped out of the bathroom, with only a towel around my waist, I ran into my sister and her friend who were just getting back in. We started talking about what they'd been up to when suddenly my sister burst out with, 'We saw you in the changing rooms, you know'.
Both girls glanced at each other and giggled and then looked back at me impudently. It was out in the open now and I realized how much I loved that and the possibilities.
'I don't really care if you did', I said. 'What's the big deal if you see me naked?'
Both girls eyes went wide at this and my sister immediately shot back, 'Well if that's true why don't you flash us now?'
I weighed this up for about half a nanosecond and then took hold of my towel, undid the knot and held it open for about two seconds. The girls reaction was priceless as they were both stunned and delighted all at once. There was no hiding my reaction this time and my towel soon bulged with what had to have been my hardest ever boner. In fact what I wanted more than anything else right then was to release it completely.
The girls couldn't fail to notice and were like sharks attracted by the scent of blood, begging me again and again to drop the towel. Eventually I just said 'What the hell' and let the towel slip to the floor.
After that I almost felt like a tour guide, showing off my prized attraction. I answered all their questions about how often and when my dick got hard, and even admitted to jerking off several times a day. When they asked me to demonstrate my better judgement should have taken hold and stopped me. But my brain, let alone my better judgement, wasn't making my decisions and I eagerly pumped away for about 10 seconds whilst their eyes took in the show.
After I stopped I told them both that I was going to my room as they probably didn't want to see my cum. But they urged me to continue telling me how they were the first of all their friends to see anything like this. That was all I needed to start again and I began stroking my dick, slowly at first but rapidly building up speed. Both my sister and her friend looked like the sexiest girls on earth at that moment and the looks on their faces were all the stimulation I needed.
My second orgasm was even more intense than the one I'd had when I'd gotten home and I struggled in vain to catch all my load in my hands as both girls unblinkingly witnessed everything. With the release of so much sexual tension I was finally able to think straight and right away I knew that I'd just given my sister an incredible amount of power over me. With what her and her friend had just seen she could blackmail me for years. And what's more I could let her.