Also, how unbelievably erotic that she's sharing the most intimate physical pleasure two people can have with a complete stranger! He didn't date her, romance her, or possibly even give her his name, but he gets to enjoy her married naked body! My wife is shared, but my biggest unfulfilled fantasy is her spreading her legs and inviting a man she doesn't even know into her body.
What an incredible experience! To have the woman you married mouth "I love you" to you while her legs are spread and up in the air, and she's clearly enjoying this other man's cock fucking her and using her beautiful body for his pleasure!