The LI/MC dynamic works in Guinevere because the authors aren't trying to sell us instant attraction twice over.
Arthur and Guinevere are betrothed. It is a political marriage but Arthur wants to make it work. The fact that Guinevere is attractive obviously helps, but Arthur was already invested in the idea of her.
Lancelot... He is attracted to the physical. He looks at her, thinks she is hot, but then he learns her name and she becomes forbidden fruit. This is also where the pre-established characters helps, because Lancelot is a womanizer. Him hitting on an attractive woman felt natural to me.
There is a foundation there and the relationships now have room to grow naturally.
















