There are versions of the story, when Arthur leaves Camelot and Mordred takes the throne, of Mordred trying to marry Guinevere. In some she does (tho I think this one also differs, whether she did it willingly or was forced), in others she flees. I haven't actually read them, but it's come up in my research of the myths. There's this article on the subject of the differing versions if you're curious.
thanks for the link, it was really educational. I knew that Arthurian tells went back almost as far as there was an England as we know it. Still I had no idea that GxM stories even existed, never mind were even older than the GxAxL stories. So cool honestly. Even cooler that you have yourself done your research and reading before writing your own work. Color me seriously impressed!