Ask for it.

"You know that thing you did last night...with your finger?"

"I hope it was okay...I mean...I know how guys are about that kinda thing," she said nervously.

"Oh god, it was amazing, baby," I told her. "I wish we'd tried it sooner!"

"I'm glad," Amy said, pressing harder against my cock.

"I was kinda hoping we could try it again...maybe with something...bigger."

She looked at me funny for a minute and I couldn't read her expression.