This happened to me in college. Halloween was coming up and I was going to a party as Tarzan. I found a loincloth at a crowded costume shop (it had a little rayon pouch underneath so you wouldn't flash anyone inadvertently) and got into the long line to try it on. The girl in front of me saw what I was going to try on and struck up a conversation with me. She was rather flirtatious, which I enjoyed because girls didn't ordinarily flirt with me.
Our chat was going really well, and when she finally got to the front of the line for a dressing room and was next, I did something bold, I followed her into the next dressing room, one in the back corner (there were six). She seemed surprised and laughed about it nervously, but I gave her a reassuring look as we squeezed into the small dressing room. While she fussed with her Star Trek costume (stalling, I think), I began to undress as she watched me in the mirror. When I was naked, my cock started growing pretty fast. I was actully pretty nervous, and fumbled with the loincloth, which had turned inside out. After putting it on, we both stood there staring at the bulging flap of faux leather in front. I have one of those penises that sticks straight up toward the ceiling when it's hard. 'Your turn,' I suggested, hoping for a show.
She stripped down to her bra and panties to reveal an almost Rubenesque figure, pale skin, large, round tits, narrow waist but wide hips, and long dark hair parted down the middle, which fell over her breasts but didn't cover them too much. As she pulled on the dress, which was little more than a long blue cotton t-shirt with a Star Trek insignia on it, it was evident that it was too tight and too short. 'God, that's hot,' I panted, and we both laughed.
She lifted up my leather flap, discovering the rayon pouch at which she made a frowny face. 'That's no fun!' she pouted, so I reached in and pulled the pouch to the side, revealing my stiff one. 'Try that dress on without your underwear...come on, it's only fair.'
She obliged by stripping naked, and before she could put the dress back on I kissed one of her tits, then licked the nipple playfully. I attempted to hold her but she struggled free and hastily stuffed herself back into the dress, sans undergarments. I loved the way the dress hugged her tits, the fabric getting hung up under them to reveal their shape better, the thin cotton doing little to conceal her nipples and bumpy aureoles.
By this time I was so horny, I took my cock in my hand and started jacking off while staring at her tits. 'Fuck, you've got a good body,' I said breathily while her eyes were glued to my cock.
I pushed her against the mirror, which had a little bench in front of it, causing her pelvis to be thrust out conspicuously. 'Let me see your pussy,' I said, with an urgency she couldn't resist, and she pulled the dress up a bit to expose her thick dark bush and two thick pink pussy lips that parted and glistened in the harsh fluorescent light. At the site of that, well, I felt a tremendous orgasm welling up from my balls. Pulling the dress up some more with my left hand, with my right I pumped furiously, aiming my cock at the mass of pubic hair. She caught on and squatted a little, using both hands to part her pussy lips thereby creating a more appetizing target for my load.
A moment later, I grunted loudly and shot several gobs of cum onto her hands, her pubes, one rope of it scoring a bull's eye between the two lips. I let myself collapse to my knees before her, and with my face staring at her cum-covered hands and pussy, I licked the cum from her upper thighs, getting cum on my nose in the process.
'Some time soon I'm going to fuck your brains out, whatever your name is,' she said, and we both burst out laughing.
'But not today?' I asked, disappointed.
Just then a clerk knocked on the door. 'Everything alright in there?' she asked.
'Yes, thank you,' my Star Trek companion responded.
We both ended up buying our respective costumes, exchanging names and phone numbers at the register, but I never did manage to arrange a date with her. I saw her about a year later at a cafe and we smiled at each other, and that's the last time I saw her.