Mark sees a dark tongue chocolate. He thinks that it might be fun to have a bigger tongue; maybe Rachel might like it. Then he remembers how she mixed two together.
He takes the dark tongue chocolate and also finds a dark penis one. He melts them together and pops them in his mouth.
Almost immediately, he feels his dick change. He's wasn't sure if his dick would change or if his tongue would, but he's glad it went this way. He looks down and sees it grow a little bit, and the skin changes to be a little bumpier. He notices that it doesn't just look like it's bigger, it looks like it has more muscle! Mark focuses and tries to find the muscle, his brain working on where to send the signal. He finally finds it and can move his dick around a bit, and not just the whole thing back and forth, but he can curve it a little bit like a tongue! He reaches down and grabs it, and he has the weirdest sensation! He can taste through it! It's not overpowering or too strong, but he can definitely taste his sweaty hand. He also notices that his dick is a little wet, and not just from Rachel's pussy; it's definitely secreting saliva through the skin. It's weird for Mark to have the sensation of taste somewhere other than his mouth.
He grabs another tongue and penis chocolate and melts them together and eats them. His dick tones up some more and gets more muscular, particularly around the base, and looks a little more like a tongue, flatter but clearly stronger. He focuses and can definitely make it move around more, including only parts of it like tip. He can almost make it touch his stomach.
Just then Rachel emerges from the bathroom and sees Mark's flexible, large penis and, while not entirely sure what he's done, is immediately drawn to it. Rachel approaches Mark and leans into him, kissing him. She reaches up and tweaks his nipples, causing him to jump a bit and his new cock get harder. She kisses down his body, stopping to lick around his nipples. Mark is getting really horny now and his dick is rock hard. Her mouth arrives between his legs and she kisses the tip of his cock, then takes the whole thing in her mouth. Immediately, Mark moans. He can taste her mouth and without thinking, moves his dick around in her mouth. Rachel feels his dick french kissing her and is surprised, but quickly gets into it. She gives him the best blowjob she can, making sure to slide her tongue around it, feeling its roughness.
Mark puts his hands on the back of her head grabs her hair. He slides his fingers down the back of her head and onto her back, then moves them around and grabs her rock-hard cock nipples. She moans a bit, making his sensitive dick vibrate with delight. He puts his hands back on her head but can't take it much more. His dick spasms and he unloads his cum into her mouth. He can taste his salty cum mixing in her mouth. She swallows it all then moves up to kiss him. At first taken aback at the thought of tasting his own cum, he realises he already did that and french kisses her deeply, tasting the exact same thing on his mouth's tongue as he did on his dick.
Rachel straddles him as they kiss, her still-hard cock-nipples rubbing against his chest. He focuses and makes his cock-tongue slide up and down her pussy, teasing her clit and making her shudder, both of her pussies soaking wet. She moans as he traces her original pussy lips with his cock-tongue. He can taste her juices. She presses down into it, but Mark is still softening and will need a few minutes to recover.
Rachel breaks the kiss and looks into Mark's eyes for a moment, as if contemplating what to do. She knows she can wait and he can please her more with what he has now while they wait for him to recover, or they can make changes. There's a momentary silence while they unknowingly contemplate the same thing.