My wife and I have that exact same metal spanking / fucking bench. We switch, and she recently bought a large strapon. She says, "if you can fuck me in the ass, then when it's your turn you have to take one bigger than your own dick." -- a little afraid, but excited too. When she's on top, always lots of pulling and squeezing on the balls. Probably next Thursday, after a lot of oral worship for her and ball squeezing for me.