This is so much more tame than some of the other posts here, but nevertheless extremely hot to me.

My GF and I were chilling at home one day recently when she mentioned her sister would be coming over. I was in the middle of a game and didn't register it, but nodded along.
Let's call my GF 'Tracy', and her sister 'Stacy'.

I generally avoid underwear around the house... just feels so damn good, and I'm a 'shower' with a fairly large penis and a voyeurism kink who likes teasing Tracy (who's a bit of a size queen and fixates on my size).

When Stacy arrived, I suddenly realised I was still going commando in a pair of tight gym shorts. I had the option to sneak off and change... but decided in the moment (horny brain) it would be so much more exciting not to.

Stacy is a very shy, dignified, demure person, rarely (if ever) discussing anything of a sexual nature. We've gotten to know one another well over the past few years, and she's very comfortable around me - treating me almost like a brother, often confiding things about her personal life that would be closed off to most other people. She's also very attractive. I admit, I often think/fantasise about what she's like behind closed doors, what her secret kinks are... if she ever thinks about me the same way. Tracy was similarly aloof when I first met her, and turned out to be VERY different behind closed doors. I believe they're both demisexuals. Secretly, I wish I could call them both my girlfriends... but that's verging on a different confession.

Anyway, there were numerous points throughout the day when I'd walk directly in front of Stacy as she was seated (with my dick flopping visibly around inside my shorts), or sit next to her on our couch with legs splayed (with my GF on the other side as we watched a movie). The outline was there, clear as day, and I'd occasionally do things on the down-low to enhance it (or her view of it). She was sitting slightly behind me as we watched the movie, so would have been at liberty to look without being caught. Needless to say I was fairly turned on the entire time and that only exaggerated the dick-print.

Stacy never made any comment and I didn't catch her staring directly at it... until the very end of the night when we were saying goodbye and I was gazing past her. While she must have seen it earlier, in the hallway to our house my black shorts were brightly illuminated by the overhead lights... which in tandem with the persistent semi-chub I had, created a very flattering silhouette against my leg (I actually didn't realise JUST how visible it would have been until I used the mirror afterwards... the shape of my entire glans could be made out, a quarter of the way down my thigh). She glanced away as soon as I turned back towards her... but that stare happened.

I know I'm an asshole for this. Part of my feels so guilty, like I flashed this girl despite being clothed in my own home. And it's definitely a betrayal of my GF (who I love very much). Buuuuut this isn't exactly a subreddit for ethical confessions. And that guiltiness is drowned out by a feeling of perverse satisfaction. I feel like a boundary's been crossed.

Stacy's seen the dick that's packing her sister, in maximum glory (or at least, the best I could've asked for). I hope she thinks about me long and hard (sorry) in her quiet hours.

TL;DR: GF's lovely sister came over. I went commando and put my dick-print on full display. It didn't go unnoticed.