I can't help myself, I want to add to the pool of all the different 'Guenevere reactions'.
My Guenevere will be sad and hurt at the news, yet her rational mind would kick in immediately (this also being her defence against unwanted emotions and painful situations) and she would try, and succeed, to forgive both of them and reunite them. Though not without a few frosty words shared (a mild argument?) with one or both, just to give the frustration within her an outlet. For she is - madly - in love with Morgana, whilst she wanted to be a good and kind wife for Arthur from the start, and had grown to be truly fond of him over the years. Thus Guenevere would never willingly part with either of them, despite this weird predicament.
Also I have got a question, or several. All which will probably not be answered since they could be easily seen as spoilers, but, alas, I can't seem to stop typing.
1. Will there be any sort of interaction between Guenevere and Lot (Morgana's husband)? Since my Guenevere is head over heals for the woman she would be... interested... to see who got lucky to marry Morgana and how he treats her.
2. Are we going to see more of Morgana's current children in future books/chapters? And do the choices Guenevere make throughout the game have any significant outcome for the boys? (besides their mother 'turning evil' or 'staying good'.)
I am sorry if these questions have been asked and answered before, but I searched for the life of me and couldn't find anything between the comments.