Today will be one of the more interesting days in the month because it talks about how Poseidon relates to gender and sexuality. This entry will be both based on classical and UPG sources. There are some deities around the world that are very particular in their approaches. In my understanding, Poseidon is not one of those.
Like all of the Hellenic Theoi (even the “virgin” Goddesses), Poseidon does not condemn any form of sexuality. This is because modern standards of sexuality were not part of Hellenic culture and the puritanical norms of Western society came about due to Christianity’s influence on social customs. In my estimation, I do not think that Poseidon would have any difficulty with homosexuality, because in Classical Hellenic society there were not the same stigmas around same-sex relationships than there are in modern society.
Although Poseidon cannot be considered gender-“fluid” in any way, because in the mythology Poseidon did assume different masculine forms for himself, I would argue that he would be open to different expressions of gender, including transgender individuals. And because of the flowing nature of his Realm, I would also consider gender-fluid individuals to be acceptable in his eyes.