It's remove inhibitions, not fundamentally alter their core personality and genetics.
The problem isn't that I don't understand remove inhibitions, but that you don't understand sexual orientation.
Remove Inhibitions isn't the spell that changes sexual orientation, that is Morph Sexuality. Every time the MC uses remove inhibitions, he basically makes the girl more likely to act on her curiosity or less likely to talk herself out of trying something because of fears and societal norms.
Evelyn was already curious about being with a girl and already in love with the MC so using remove inhibitions to convince her to have sex with Callie and the MC makes sense.
Primrose was curious about a lot of sex acts, but because her dad told her that certain acts were naughty convinced her not to try them. So using remove inhibitions to undo what her dad did makes sense.
But Callie being a lesbian required a different spell (Morph Sexuality) to make her ever so slightly bi-curious. Remove inhibitions wouldn't work here because it wasn't an inhibition. Once she was slightly bi-curious, the MC has to slowly let her experiment with him so she will become bisexual.
The problem with your view is that you don't understand what part of each character's behaviour is caused by inhibitions to be removed and what is caused by personality and genetics.