By rubbing the front of his panties

"Have you ever seen a girl masturbate, Robin? They don't do it by grabbing and stroking their clit like you just did. Pull your panties up."

Robin did as he was told, even though his cock was still hard. It tented his feminine panties drastically.

"Good, now rub the front of your panties with the flat of your hand, just like a girl."

"Right now? I'm still really close to cuming."

"Yes, right now, I want you to stay on the edge. Don't worry, it will be a little harder to cum by mistake this way."

Robin did as he was told, rubbing over the front of the panties stretched over his cock. "Good, just like that," Dr. Clay said, "the feeling of the silk makes you feel sexy and feminine. It will feel even better if you moan like a girl while you do it."

Robin started to moan in a high-pitched feminine way, almost like he was imitating an anime character. Dr. Clay made sure to catch the humiliating display on his camera.

"Good, now a little faster."

Robin started rubbing faster and Dr. Clay noticed a growing wet-spot on the front of his, panties from his pre-cum.

"Are you feeling close, Robin?"

"I...I don't know. I'm not sure I can cum this way. It feels good, but I just want to grab my clit and stroke, like before."

"No, you have to resist those masculine urges if you're ever going to get better. In fact, there's a way we can make your edging even more feminine..."