We'd taken sexy photos before. She was an enthusiastic model and loved the excuse to buy and show off lingerie. We'd hang a white sheet for a backdrop and she'd step into the role of model and I'd fulfill the role of fashion photographer.
Once, when she was modeling an especially sexy white outfit (bra, thong, garter and stockings) she wanted to do something new. She looked around for a prop to incorporate, and seeing the red butt-plug, decided to use that.