My girlfriend Sophie and I were house-sitting for a friend of hers I’ll call Sally. We slept in her bed and fucked like crazy. 

I have to admit I was thinking of her slim attractive girlfriend taking care of her own urges in this very bed, while we were doing it. 

When Sophie went to take a shower in the morning, I couldn't resist sneaking a peek in Sally’s knicker drawer. There was the usual selection of day-to-day panties, a couple of thongs for special occasions, and deeper down a pair of red silk ones. I picked them up, half thinking I might borrow them, maybe take them to the bathroom and use them to masturbate with, but surprise surprise, out popped a dildo. It was a realistic cock shape and pretty big. 'You horny minx,' I thought.

I was so absorbed by this discovery I didn't hear my girlfriend come back into the room and I was caught red-handed.

"What the fuck do you think you're doing in Sally's drawers?" she demanded. 

I hastily dropped the panties but the dildo fell with a clunk to the floor.

"Oh my god! I never thought she'd have one of those!" squealed Sophie. 

She reached for it, and half-joking put it to her nipple, which had fallen out of her bathrobe. Turning the knob on, the dildo sprang to life and began to vibrate on her sensitive tit. 

"Ooh, I get it now!” Sophie shuddered, and tried the other nipple.

“I’ve always wanted to try one of these but I've never had the nerve. Where did you find it?"

"It was wrapped in those red panties," I confessed.

"I bet she uses those to get off too,” Sophie breathed harder, scornfully picking up the shiny red satin and fingering it.

"I dare you to try them on.”

"That would turn you on, wouldn't it? I know you really fancy her.”

"Well, she is rather pretty in a petite sort of way, don't you think?"

“So you say, I quite fancy her too, not that I would ever do anything about it…except perhaps in a fantasy."

The whole idea of this was really turning me on. 

"Go on, I dare you.”

"I'm not sure they will fit me, but..."

Sophie was being unusually compliant. She was obviously as excited by the idea as I was. 

She let the fluffy bathrobe fall to the floor, exposing her lovely curvy body, then stepped into her girlfriend’s favourite red panties. The ones she must have used to dildo herself in. They were pretty tight over her hips and super snug over her cunt, little wisps of pubes poking out of the sides. 

“What do you reckon?” Sophie gasped, admiring herself in the mirror. 

"I wonder what she does with this?” she mused, picking up the dildo, which was still buzzing on the bed. "I bet she does this. Oh well, here goes.” 

Sophie lay back on the bed and parted her thighs, allowing the vibrating dildo to trace its route over her friend’s knickers. I could see a wet spot beginning to form on the crotch of red satin pulled tight over her plump cunt.

"That feels fucking horny, actually!”

Sophie didn't usually use dirty words, but the borrowed panties seemed to have an effect upon her. Probably the same effect they had on her friend Sally. 

She teasingly rubbed her mate’s dildo back and forth over the tight satin crotch, pushing its pulsing tip into her slit, which was clearly outlined in the red silk. I knew it was turning her on doing it with her sexy friend’s personal toy. It was naughty and transgressive, but she was doing it anyway, and had passed the point of caring. I don't think she would have stopped even if her friend had walked in on her.

"I see this is turning you on as much as it is me, looking at your hard-on,” Sophie grunted. “Why don't you do something about it?" 

I didn't need any more encouragement and placed my straining cock over the red satin, but almost as soon as I pulled my foreskin back over the head of my cock an arc of spunk shot out over Sally's panties, staining them cream and dark. 

With that, Sophie managed to pull aside the tight crotch and plunge the dildo hard up into her cunt. The same cunt where earlier that morning my cock had been, fucking her and thinking about her best mate. The same dildo which must have been deep inside Sally.

That did it for her. Sophie plunged the dildo deeply and repeatedly into her sopping cunt with one hand, pulling on the knickers with the other, and coming much faster and harder than I had ever seen her do before.

I was left to finish myself off, spurting repeatedly over her friend’s red satin panties.

When she collapsed back onto the bed, her milky thighs spread wide, the panties stained and ruined, she just smiled at me, all guilt gone. The dildo plopped out of her cunt and onto the bed.

"I think I've discovered something here. I wonder if she'd miss them?” she mused.

"I think she definitely would!” 

"Shall I tell her or just put them back as they are?” 

"It's up to you."