He gestures, uncertainly with his left hand and it reveals part of his absolute conviction that if this happens it will end in pain and yelling and loss of friendship. And he's already loathed by Shakespeare (or rather, Puck, who sort of represents all of the Shakespeare canon to Darren) and doesn't think he can deal with Marlowe too. It's just this stupid mess. He's not having sex. He's on romantic probation.

"It'll be messy."