Brats like her will always learn to be honest with their masters, sooner or later. Especially with a little... incentive to keep her from denying her more primal instincts.
Hinata's acting like I won't knock her up too, it's a white man's job to knock up as many females as possible, there's no point in just stopping at just one baby factory when there are millions out there
Most people in Chaldea are always wary of Raikou, especially the Japanese Servants. She's a Berserker and the legendary Oni Slayer, after all... Which is why nobody could have expected this outcome. Not even five minutes after meeting her new White Master and she was already on her knees, getting her mouth filled with a BWC. So much for being the protector of public decency.