The Alexandrian solution

The Alexandrian solution - Bexless Hilarious!

Stiles, Derek, knotting.

In this one the guys are already in an established relationship. They are very surprised to find something is different about Derek’s anatomy.

"Seriously," he says, still staring. "You couldn't warn a guy?"
"I told you," Derek grits out, hands on his hips. He's still staring down at his crotch. "I didn't know."
"How could you not know?" Stiles bugs his eyes at Derek. "I know you've had sex before. You've had sex with me before!"
"This never happened any of those times."

"We can research it," Stiles says happily. "I wonder what else freaky shit there is we don't know about yet? Do you think anything else is going to happen to your dick? Maybe it's evolving into something else! Like a Pokémon!"
"We can only hope," Derek says.