So, this is sort of a headcanon? A bit of an exploration? I feel like it'll ramble on...
I was thinking that the Guinevere who has an affair with Mordred/Lancelot/Bedivere is usually Arthur's third wife and also the one who tends to be abducted a lot. My brain always kinda figured that maybe after one kidnapping too many, Arthur got scared of losing her and changed, becoming maybe a little too controlling and protective of her.
She spends a few years, practically a prisoner, in Camelot with an Arthur so different from the man she fell in love with and he won't listen to her. She's virtually alone with no one who understands (or is even capable of doing so) and she's unsatisfied with herself and life in general.
She seems to begin every love triangle by being introduced to the other man and barely giving him the time of day. Cue being kidnapped one final time.
From here, if the suitor is Mordred, she continues on in faithful misery and he pines from the shadows. Arthur leaves to fight Romans and then... there's Mordred with a comforting shoulder and all the love and respect in the world she could ask for; Arthur's gone and here's a man who gives her the one thing she craves above all: autonomy.
In stories with Lancelot, she takes him to bed while Arthur is still there. I figure it's sort of his constant presence and concern for her that drew Guinevere into his embrace quicker. Lancelot's heart compels him to listen to her (like Arthur won't) and she finds freedom with him: she's choosing this, choosing him, and she's able regain a bit of her old self by doing so.
In the tale where her affair is with Bedivere, who he is adds another layer of complexity other than simply replacing Lancelot. Because she knew him. For years. From nearly the very moment she met and married Arthur, Bedivere has been there, the faithful witness to Arthur's shift in character and Guinevere's own diminishing. One of Arthur's best friends from the early days reaches out to his friend's wife and queen, likely in the hopes of fixing their marriage (like a good bro would), but winds up falling in love with her himself. From there, Bedivere stands up a crux: betray Arthur, his brother, his friend, his king? Or condemn himself and Guinevere (who he now loves) to continue ignoring each other and be drawn further into misery? The two choose each other and Guinevere gains while he takes a moral loss.
I don't think Guinevere is justified her in adultery, but I do believe she's understandable in why. I'm sorry this came out like an essay, but thank you for reading.
