I also didn't need to learn that Duncan really doesn't like gay people, that seems like the kind of fact that just makes Duncan seem like an unlikeable a-hole. Its not even like it was a throwaway comment, its a full conversation that just made me wonder what the point is. I guess it was more of FH's philosophy, but I just don't see the point. If it was philosophy, I'm not sure what side FH was trying to support, either.
I'd have to reread that, but I think part of it was that Duncan himself was heterosexual, and during his time with the Fremen, he internalized much of their culture and attitudes. Homosexuality/lesbianism were, to the Fremen, anathema. As in any tribal culture, people who refused to conform, refused to do their part in reproducing the next generation, were detrimental to the tribe's survival, and therefore were not to be tolerated.
As for FH's own opinions, who cares?