Clothing is an armour for both of them, in different ways. The collar is open because Oswald opened it and liked it and Riddler wants Oswald's eyes trapped on him.

"I don't sleep well. Or a lot." It's a warning, not disagreement. "But I'd feel better knowing you were there. That I'm not going to wake up and think I'm in Arkham. And you're allowed to just touch," Riddler adds with a smile. "To know I'm really here."