As I've commented before:

I'm not so keen on boycotting [people or organizations] that don't incorporate their objectionable political or social views into their products or services. I think there's an important distinction between "paying people who hate me" and "paying people to hate me". I don't mind seeing a movie which stars an actor who has abhorrent political views, as long as those views don't work their way into the movie. I don't think that buying a product from a person indicates, or should be commonly understood as indicating, that the buyer likes the seller or the seller's political views. I think that being willing to interact with people who we despise (and people who despise us) is important for living in a pluralistic society. I don't want cancel culture turned in my favor; I want it dismantled completely.

I think this reasoning applies in this case, with a couple of caveats. If the trans person doesn't pass (in terms of both appearance and voice) as an ordinary member of the sex that their character is supposed to be, then I'd consider it to be politically motivated casting, and so I'd refuse to see the movie. In addition, if the character they're supposed to play is trans, then I'd probably consider that to be politically motivated writing, and so I'd refuse to see the movie.