Have you read any Famous Five books?
If TV series count, how about Holly in Red Dwarf?
How about the female character inhabiting John Malkovich’s body during sex in Being John Malkovich?
IIRC, that was a different Holly, swapped with the original during an alternate-worlds episode.
Didn’t they end up getting the “original” Holly back, but he decided he liked the new identity (at least for the rest of that season)? Or am I confused?
This is one of those times where I know I’m not answering in the spirit of the OP, but figure I may as well give this example anyway as it may inspire interesting discussion.
There was the TNG episode “The Host” where Dr. Crusher fell in love with Ambassador Odan, who was really just a host body for a Trill. Odan gets hurt so the Trill is transferred to Riker, (which caused Crusher to put up what only her mother would consider a fight), but then was transferred into the true next host body, which was a woman’s body.
Of course, the episode then completely pussed out and had Crusher find that situation “icky” so they went their separate ways. Nice message there - people will still have close-minded sexuality issues 300 years from now in a multi-species Universe.