Jennifer Ashley has a series of books about a Scottish family called the Mackenzies. I've only read the first one, but it was very good and had a lot of interaction with other family members.
I've read several Lora Leigh books with this. It happens in the first chapter of Dangerous Pleasure. (LOTS of content warning on this one, ranging from racial issue to kidnapping.) I think it also happens in one of the early Breeds books. The Man Within maybe? I know Taming the Beast has the opposite scene, where she catches him masturbating.
All of them are erotica books.