In most game they don't really become bi though, they just start having threesomes for the amusement of the MC, which is not the same thing. Also straight is the "default" sexual orientation everyone assume you have, it takes some soul searching in order to figure out that maybe it's not the case for you. So a straight woman finding out that she's actually bi/lesbian is a lot more believable than a lesbian woman suddenly realizing that all this time she's been depriving herself of a man for nothing.I've noticed almost all the VN porn games have straight women become bi at the flick of a nipple, and most accept this, but they seem to get irate at the thought that a lesbian might decide she's bi. Never understood this
Also in this case it's not Callie realizing she's attracted to MC, you're literally using mind control to forcefully change her sexual orientation, so she sucks your dick - no free will invovled. I can see why some people have a problem accepting that.