This happened years ago, but I still use it for one of my masturbatory pleasures.
My wife was out of town visiting an ailing aunt, leaving my daughter and I alone for a long weekend. My daughter was a middle teen and very well physically developed, having started puberty at 11 years old and developed rapidly into a full bodied figure with large breasts, wide hips and a lovely round ass. Older high school boys noticed her and were more or less a constant presence at our house.
On the weekend my wife was gone, I had another opportunity to see my daughter in her baby doll pajamas in the mornings and when we watched TV in the evenings. She was always quick to don her pj's after dinner, and I was sure the she knew it turned me on. I had many nice glimpses of her ass and her well rounded 34C titties. Yep, I used to look at her bras and panties when home alone, and often used them to jerk off with.
On the Friday night of our week end we stayed up late, and I was in a more or less constant state of erection with her almost see-through baby dolls.
The phone rang around ten, and she almost dashed to answer it before I could pick up. I figured it had to be her boyfriend, so I left the room so she could have some privacy. After she hung up, she said that she was tired and was going to bed, so I turned off the TV and went to bed also, knowing that I needed badly to masturbate after seeing her big titties and ass outlined by her skimpy pj's. It didn't take me long to cum and then fall asleep.
Later, I was awakened by hearing a door or some noise. I checked the clock and it was after midnight. I got up and walked down the hall and saw my daughter's bedroom door open, so I looked in and found her bed empty. Hearing something outside on the patio, I looked out the window and saw my daughter and her boyfriend in a big clinch, with him fondling her ass. I immediately got a hard on and was transfixed as I then watched them moving the two seat patio couch out of the moonlight into the shadow right under the window.
Her boyfriend was a senior and a hunk, as she referred to him. He was a good looking kid, a good athlete and good student. So, I could see why she was attracted to him, he being a senior and she just a sophomore. They immediately went into a clinch, and his hands were all over her well rounded titties. Then she pulled up her top and bared her breasts. I knew from seeing her bras they were large for her age, but seeing them bare got me as hard as ever I had been as a high school kid. Full and round with hard pink nipples, there was no droop or sag. She took her top off as he started to kiss them and suck on them. I couldn't hear her, but she was obviously squirming and moaning. Then he reached down and started to rub between her legs, which quickly opened to his hand. She reached over and began massaging his crotch. Quickly he opened the fly of his shorts and pulled them down, and out sprang his 18 year old cock, stiff and curving up at the night sky. She started slowly rubbing him, demonstrating to me that this wasn't the first time she had caressed it.
She then put his hand inside her baby doll panties and whispered something to him.
I was afraid that they were going to fuck, and I didn't know if he had a condom or not. But, as it turned out, I needn't have worried. It didn't take long until I saw her arching her back and thrusting her hips into his hand. She was obviously reaching orgasm, and I was almost there, too. It must have been a good one as her whole pelvic area thrust up and down as she convulsed into orgasm.
Then, he whispered something to her, and she leant over and hesitantly started to rub his enormous hard cock. Then he sort of gently pushed her hand down, and she rubbed harder. It didn't take long before he started to jerk into orgasm. I, too, was bringing myself off.
I never told her that I had seen them; but shortly afterwards, I bought a box of condoms and gave them to her and made her promise never to have sex without her boy friend using them in case he didn't carry them with him. She blushed like hell, but she smiled and took them. I did tell her that I thought her tits were bigger and rounder than her mother's. I don't know if she ever told her mother, but I strongly suspect that she didn't.