Yeah I agree though it is a bizarre change none the less since we've basically flipping the words in this case. The part of me that wants to do it is purely out of the sheer ridiculousness that this would entail as well as what would follow, and the fact that this whole series was based off a partly eroge doujin in the form of Fate/Stay. The more professional roleplayer part of me sees this as out there, and unneeded. This was a one off joke but we didn't consider the fact that Leon is at the stage in life, and Khan is open to this sort of things maybe a bit like Nero would be so we found ourselves here.
The real complications from where this would leave us is we'd need to cut Leon, and Rider out for the rest of the night, and summon Mordred the next morning, or whenever. Rider's master will need to deal with this fact as well but according to Grey thy wouldn't care all too much if they were being serious. If brought up Mordred will likely find issue with it due to her often close attachment to her masters be it as a father figure, family, or possibly more.
So yeah that's basically the jest of this whole deal. A joke that has the potential to lead into something else.