Probably the most exciting and pleasurable sex I ever experienced was with my sister-in-law and, get this, we never touched each other. This was over thirty years ago. My wife and I were in our early twenties and had been married for only a short time. We didn't have a lot of money for entertainment in those days, but for several years her sister and husband (who were about ten years older than us) would bring a 12-pack of beer over to our apartment and we would play cards or board games all evening.
The sister-in-law wasn't movie-star-beautiful, but she exuded sensuality and lust. Sis had no children due to her husband being sterile. She had a small frame, with huge pendulous breasts and a well-rounded bottom. Those were the days of mini skirts and she would always seat herself across from me while we played cards. As often as I could, without arousing my wife's suspicion, I would 'accidentally' drop a card on the floor and sneak a good look up her skirt. Sis knew what I was up to and would open her legs wide to give me a good view of her creamy thighs and the thick bush of pubic hair poking out around the crotch of her panties. This would give me an instant erection and it was difficult to hide my state of horniness from my wife and Sis's husband.
I don't remember exactly how it evolved, but as the days and weeks of these visits wore on, Sis and I secretly devised a way to release our sexual tensions. Because we all drank a lot during the card playing, it seemed that one of us was always heading to the bathroom to urinate. We worked it out so that on Sis's first trip of the evening she would sit on the toilet and masturbate herself to orgasm through her panties. She would then remove the panties and hide them under the vanity before rejoining us. A while later I would go to the john and retrieve her panties. Fondling them, I'd bring them up to my face and inhale the intoxicating musky fragrance of her cum juice. Then I would rub the head of my rock hard boner on the silky material and ejaculate several large stringy gobs of my spunk into the crotch. I can't describe the intense pleasure I experienced in those orgasms. I would then put the panties back in the hiding place, clean myself up and go back to join the others.
Later on Sis would make another trek to the bathroom, fetch the panties and pull them on. This is where she really got her jollies. Using my spent load as a lubricant she would finger her slit and rub her clitoris through the panties, giving herself several intense orgasms. Years later she confided to me that it was the lascivious nature of these acts that really turned her on. She said that her utterly sluttish behavior served as a most powerful aphrodisiac. Also just thinking about the risk of being impregnated by my live sperm (which would be drawn further toward her fertile eggs each time her vaginal muscles rippled in orgasm) excited her so much that she was unable to stop herself from cumming repeatedly.
The real payoff was when she returned to the table, noticeably flush in the afterglow of her release, with only she and I sharing the knowledge that her panty crotch, now heavily loaded with our mingled juices, was squishing into her sopping wet opening. The randy scent of sex would permeate the room, and it's still a mystery why my wife and Sis's husband never caught on to our little game.
When Sis and her husband would get ready to leave for the evening, I would still be on a tremendous sexual high, and after they left I'd ravish my wife unmercifully. She loved the attention, but never did make the connection as to why I was always so horny after her sister visited.