bizarre 80s-90s movie ,in the end this guy is meant to marry this girl that's like a freak/demon .?

It sounds like a much-later movie:

Dagon (2001)

starring Ezra Godden, Francisco Rabal, Raquel Meroño, Macarena Gómez, Brendan Price

IMDB synopsis:

Based on a short story by H.P. Lovecraft, the undisputed master of the macabre, "Dagon" tells the story of Paul Marsh, a young man who discovers that the truth will not set him free instead it condemns him to a waking nightmare of unrelenting horror. A boating accident off the coast of Spain sends Paul and his girlfriend, Barbara, to the decrepit fishing village of Imboca looking for help. As night falls, people start to disappear and things not quite human start to appear. Paul finds himself pursued by the entire town. Running for his life, he uncovers Imboca's dark secret: that they pray to Dagon, a monstrous god of the sea. And Dagon's unholy offspring are freakish half-human creatures on the loose in Imboca.

IMDb review:

"Long ago," the priestess explains to Paul, "he was young and handsome for a man. He brought women to Imbocca, including your mother. You are my brother, now you will be my lover." Paul keeps cranking up the wench only to learn he has gotten there too late. Now covered in a thick black substance, Barbara begs him to just let her die. At first he refuses, but the priestess begins to explain how Barbara is now part of Dagon, and so he lets Barbara go into the deep. Turning to the priestess with eyes full of anger, Paul douses himself in kerosene and sets himself on fire. The priestess shouts, "No!" then leaps onto Paul, knocking them both into the water. Once underwater, gills open up in Paul's neck and sides. The priestess smiles, swimming around him just like the mermaid in the dream. Now, realizing that he is one of them, Paul takes her hand, smiling, and swims with her into an intricately carved tunnel under the sea.