She's too embarrassed to continue

She looks up at Diana, pausing her strokes "I don't think I want his semen on me, do you have a way to catch it or something?"

Diana nods and quickly reaches down, removing Sophia's hand from your cock and beginning to stroke you herself. Your friend looks you in the eyes, all business.

"Is this ok, Joe? I know we're friends so if this is too awkward, I can stop. But I know it can be uncomfortable to be close to orgasm and have someone interrupt it in the middle."

The thrill of her touch fills you with undeniable pleasure. "It feels incredible--" you respond, barely able to form words.

"I know," she responds, smiling a bit, but are you ok with me getting you off?"

"Yes," you nod, and in response she begins stroking you in earnest. Her touch is much more confident and expert than Sophia's. She begins to explain to the class what she's doing:

"When a woman is masturbating a man, she will usually begin at the base of his penis. She can use a stroking motion, like this, or she can just hold it, using her thumb to rub against the head. She can also use her free hand to stimulate his balls or stroke his thighs."

Sophia watches intently, trying to soak in the lesson.

"What's important," Diana continues, "is applying just enough pressure to his penis so that it feels pleasurable, but not so much that it is painful."

Sophia nods, looking down at your cock.

"If she's doing a good job, it should be obvious when the man is close to ejaculating. She should start stroking faster, with more intensity. She should also start squeezing his penis, either gently or firmly. She could also move her hands in different directions along the shaft. The purpose here is to create tension. When you're ready to orgasm, it's incredibly pleasurable to have something pressing on you, even painful, so that when you do orgasm it's an overwhelming relief."

You moan as you feel her squeeze the shaft of your penis.

"Make sure you don't squeeze too hard--"

You don't have time to hear any more of her instructions, as your cock throbs and you feel yourself about to cum. You're completely lost in the feeling.

"I'm going to cum!" you shout and she wraps her hand around your cock, stroking it firmly.

"It's ok, Joe," she says as she pumps you, "we're just friends, this doesn't change anything."

The pleasure in your balls overwhelms you, and you feel yourself explode. As your semen spurts out of your cock into Diana's hand, her rhythm slows.