Tbf a lot of this is because the writers themselves heavily imply many servants have feelings towards the MC. Kiichi Hogen wanting to make you her bride(even if you're a dude, shit got me actin up), Boudica saying she loves you outright, Raikou saying she loves you as a child(meaning she loves you romantically), etc.
Ritsuka basically having a thinly-veiled harem of mythological figures is easily one of my favorite things about the game because it's just so fucking out there to be banging Nero Claudius, Joan of Arc in a swimsuit, and a literal fucking DINOSAUR