Kuniko.

"Well," Diana says to the class, "You've seen male arousal, stimulation with the hand, and with the mouth. You've seen intercourse, and you've seen a female orgasm. So, the main thing left for the class is for you to see what happens when the man orgasms- specifically the penis, and how it releases semen."

Sophia's nude form, splayed out on the floor, coated with perspiration, is easily keeping you hard as Diana speaks.

"Kuniko," Diana says, "Since you're already up here, would you like to help Joe orgasm?"

"Sure," the girl shrugs, "What should I do?"

"Joe," Diana says, "What would work well for you?"

"I'd really love to see Kuniko's breasts," you say, "Maybe I could play with them while you manually bring me to climax?"

Diana hesitates, "That's not really my role here, Joe."

"Well, my thought is that you're the only one with the experience to know when I'm about to climax," you respond, "so you'll be able to alert the class."

Diana nods hesitantly. "That's fair. Kuniko, are you comfortable with that?"

Kuniko nods, and begins unbuttoning her shirt. Her black bra is quickly revealed, covering a lovely bust. She reaches behind herself, hesitates for just a moment, then removes her bra. Your cock stands at attention as she moves closer.

"I can touch you?" You ask, and she nods with a smile.

You reach out and begin to gently massage her breasts. At first she seems tense, but as you gently knead them, she lets out a sigh. Diana seems to take that as her cue to begin stroking you.

Your friend slowly glides her hand down to the base of your shaft, grips firmer, and then glides her hand back up. You moan and squeeze Kuniko's breasts harder. She flushes a little as you squeeze her, and you begin to play with her left nipple. It instantly goes erect, and you must be getting bigger as well, because Diana begins to speed up her tempo.

Kuniko's breasts are fully flushed now, and her eyes are glazing over. Charged with desire, you lean over and take her breast fully into your mouth. She gasps, a breath that turns into a moan.

"Come close, everyone," Diana says, "See how his penis is getting bigger and redder? He's getting close to orgasm. You can also tell by-"

Her sentence is interrupted by a loud cry from Kuniko, arching her breast deeper into your mouth. Without thinking you take a free hand reach between her legs, massaging her pussy through her skirt. She jumps in surprise and then melts against you pressure, grinding her damp crotch against your hands and crying out again.

You squeeze firmly and she cries out again, again, faster and harder until you realize she's also cumming. Diana speeds up her pumping, but you're too focused, sucking Kuniko's nipple, hard, as she screams in ecstasy. Finally she goes limp, and collapses back into her chair, eyes wide with surprise.