I'm hoping someone can help me out here. A while ago, I started reading a story about a guy and his girlfriend/wife. He was in to chastity and diapers, and while they weren't her thing, she played along. Their rule was either of them could choose to put him in to a cage and/or diaper, but only she could decide when he could be taken out of them. Usually, if he was in a cage and diaper, she'd tease him a bit to get him worked up, then unlock him and they'd have normal sex. However this time, after teasing him, she pulls out a strap on, has him put it on, and has her way. She tells him something like "just because you're locked up, doesn't mean I shouldn't get to get off", and uses him to pleasure herself. The story spends some time on the mindfuck of how she's riding "him" but he can't feel her like he normally would. He can feel the dull pressure of the strapon, and can see it going in and out of her as he usually does, but instead of relief, he just gets more and more pent up.
I've been searching high and low, but with no success. It absolutely mashes all my buttons, so I'm hoping I can eventually find it again.