I always thought if a guy became a girl I dont see why they would suddenly be into guys, as usually happens in such stoires. They would surely still be into girls- assuming they were in the first place.
It's because we don't know what would happen, and there are some cases of actual trans people shifting thoughts, but that's people using some to conclude for all. For all we know, those sexuality shifts could be them not internally lying to themselves subconsciously or actual hormone changes change who you like, and it could be either or both that are true. We just don't have the information, and brains are notoriously hard to understand.