I'm put off with the level of artistic license.
Shouldn't be.
It was all rationalised by the Church, though, from the original oral tales.
Guinievre would have likely been another chieftains first wife or fav concubine whom the Arthur-type would have taken, whether she liked it or not.
Lancelot was perhaps his nephew, his sister's son.
In one of the first written texts, Lancelot do Lac, he was definitely more blood-related to the king on his mother, Evaine's, side. But even that was all rationalised.
Far earlier written texts are just pieces, not very whole. But in one that I translated from Old to modern French there features a scene in which sweet Lancelot severs the top joint of his pinkie prising open iron bars so he can get in to shag Guinievre.
Arthur had placed a load of flour all over G's bed, so that any man who walks there will leave footprints. So Lancelot jumps from the window into her bed and then back again.
But he left a trail of blood, and all over the bedsheets, too.
Busted.