I think I know why Ami said Sir Lancelot, at least from what I remember (fate/stay knight, I don't anything about actual king Arthur legend) slept with King Arthur's wife. Basically Ami is Arthur, while Hayato's is Guinevere and Rei being Lancelot since she's trying to Hayato to notice her. Feel...
I am no one to judge, but even I got to admit, dressing up a stranger in the clothes of the dead boy you use to have a crush, just so you can fuck his brain out seems odd. The word boytoy seems appropriate.
I know, I'm joking.