Cole's jaws clenched hard as he tried to refrain his excitement, but Sam had tipped him over the edge. A strong spasm ran through his balls and cock, and a gush of his mutant semen exploded in Sam's mouth. There was only so much she could gulp down before choking on the obscene amount and coughing, letting go of his throbbing member. Cole immediately covered himself in the folds of the skirt to hide the mess.
"S-sorry! I swear I don't come so quickly normally", Cole lied, pressing the tent against himself but unable to conceal the growing wet spot on the front. Sam was finally catching her breath, wiping the cum on her chin. A rope of fluids had also landed on her own skirt, which was covering her softening dick.
"It's okay, I guess you were really excited after all..." Sam began, but her voice trailed. "Wow, I don't feel so good..."
Cole immediately realized his oversight. His friend had certainly ingested enough of his cum to actually turn female!
Sam was panting heavily and winced on all four in front of the bed. Her body shivered and spasmed, and Cole's eyes grew wide. It was as though Sam was starting to fill her clothes differently: while they were slightly too small and ill-adjusted before, as the two were slightly older than Cole's sister Melissa, now it looked like they suited her a little better, her shoulders no longer stretching the top over the seams, and her hips filling the skirt perfectly under a feminine, narrow waist.
The two small buds on her chest then started pushing outward under the sheer crop top. At first they formed conical mounds, but very quickly became heavier and rounder as they grew. The dark areolas and nipples also enlarged at a rapid pace, showing even more visibly as the fabric stretched to accomodate the rapidly swelling assets.
"Cole! Is... Is that from the swimming pool? Or is that you..?" Sam whimpered in a girly voice as the thin fabric began to easily shred. Her breasts escaped the confines of the crop top as it tear in pieces, and the gravity made the large orbs jiggle for a second when they fell into place.
Another spasm made Sam wince again, and she begam rummaging under her skirt.
"Oh shit oh shit..." she let out under her breath as her hands could not find any masculine package anymore. Instead, the panties she had put on were now covering a fairly moist pussy. "I'm... I'm a girl! I mean I'm physically a girl now! How is that possible?" Sam exclaimed in a decidedly feminine voice. Cole gulped audibly.
"Well, uh... I meant to tell you but it all happened so fast... There's a side effect of my cum, and it's very temporary so you don't have to worry about being stuck like that! Everything should be back in, err... maybe 12 hours at most? I hope you're not mad..."
Sam looked at Cole with wide eyes, before jumping in his arms on the bed.
"Y-you don't understand Cole, this is the best day of my life!" she sobbed as he was hugging her back. "This is what I've always wanted, and now you made it possible. Wow. I can't believe this is real!"
Cole could barely believe it, either. Which is to say, he had experimented maybe once or twice before with this on his own, so he knew it was possible, but that was quite a different situation than having one of his bros physically turning female on the same day as her coming out as trans.
He hesitantly gave her back a gentle squeeze as she was cuddling up against him. She felt slightly smaller than before, though she absolutely was the same person. Even her red hair was identical, an undercut style with a kind of messy top, which incidentally now gave her a cute butch lesbian look.
However Cole could no longer feel any bulge frotting against his own, and his own growing breasts were now pressed against Sam's fully developped chest - which while probably not much larger than C or D cups, looked huge relatively to their still young frames and twice as big as his boobs.
Sam adjusted herself against Cole, and his half-soft member caught in the skirt slid between Sam's legs and lodged itself right against her panties between her thighs, making her instinctively squirm.
"Aah fuck..." she moaned softly, "w-why am I still so horny?"
Cole let out a low grunt of frustration. His semi was not fading either. "I guess that's... another side effect..." he managed to articulate. The strong pheromones of heat emanating from Sam's pores were making him light-headed, and he felt his dick harden a little again.
Sam pulled on Cole's cardigan's strap to open his top, exposing his breasts to rub her chest against his. "Fuck, Cole... this is the best feeling..." she moaned in his ear as the soft breasts poked into each other's, their nipples very erect. Intuitively, she started slowly grinding against his erection.
The panties were drenched in fluids at this point, and Cole was smearing more against Sam's inner thighs. As the frotting pace incrased, he felt the tip of his member become pointier again and get caught on one of the leg openings. Inevitably, after a few back and forth his dick forced his way through the opening and the glans frotted against Sam's wet virgin sex.
Sam let out a lusty moan, rubbing herself even more feverishly against Cole, and she pressed on his cock with her full weight. The lubricated glans pushed her stretching hymen aside, popping right inside the opening of her vagina - and then some, Cole's thickening dick progression only slowed by the panties still in the way. Still, he could feel the texture of her pussy's walls as they tightened around his tip, and it was driving him crazy.
Sam looked at Cole with pleading eyes, and was about to say something when they both heard Cole's mom's car parking in front of the house; the car doors opened and then slammed.
"Fuck! My sister and my mom are already back!" Cole yelped in panic while Sam was still forcing herself on his throbbing cock, almost down to his returning knot. Her panties cracked a little from the girth, which forced another inch forward, and Cole felt his knot slowly swell against the warm entrance of Sam's brand new dripping wet pussy.