You didn't read the books did you? You couldn't be more wrong here. He literally only has his heirs with Chani. He has a child with her before ascending to the throne which dies. He then has twins with her that become the protagonists of book 3 and 4.

In the movie it's made less clear that he only has a diplomatic marriage with Irulan and doesn't intend to have sex with her. I assume since they plan to adapt Dune Messiah which requires him and CHani to reunite and have their children, that the movies will join back up with the book.

But he does not have sex with Irulan. He is married in name only, for the sake of the monarchy. And Chani is the only person he ever has sex with and is the mother of all his kids.