Given Heinlein's life it seems a little odd to hear him described as a fascist, especially when you've got men like Asimov describing him as a "flaming liberal" from Heinlein's early life.
I think he's written widely, and encompassed a lot of subjects from all kind of angles, and some readers seem to have read some of those through non-perscription specs. For me he writes with three main motivations or components; stimulating ideas, mechanical/physical accuracy and the desire to sell a pile of copies. I see nothing fascistic to that.
It's probably fortunate that no-one's touched on any of the sexual themes he's expressed. Jill's thoughts on homosexuality in Stranger in a Strange Land have been labelled homophobic but without regard for charactor's relatively neutral thoughts in The Moon Is a Harsh Mistress and their positive thoughts in I Will Fear No Evil. None of this charactorisation neccessarily reflects the author's position.
If you look at a book published around the same time as both SiaSL and ST, say To Kill a Mockingbird. It features some harsh charactors, and a grim situation but could we claim Harper Lee is a racist because she features bigoted charactors, or is she defending the act of rape, or even the scapegoating of black men for judicial and social convenience?
Of course we could claim this, but we'd be wrong. Which makes me wonder why folk make the effort to make harsh and unsupportable claims against well established and highly regarded writers. The web seems stuffed with such claims, matching the conspiracy theories for both width and shallowness.
Perhaps it boils down to two major parts, the first a form of bandwagon jumping, because it's much easier for one's theories to be found and read when attached to someone else's fame, which brings us to the second; it's much easier to do that than compete with and better those you're slating.
It's a worrying development, a mix of claimed moral superiority, a puritanical judgement on the quality not of the literature but the writer, supposition of political bias and the attempted blacklisting of books which might contain "ungood" charactors.
I would draw a comparison with Fahrenheit 451 but as Ray Bradbury was a queer beating, baby eating, woman hating, anthrax snorting, kiddy fiddling, member of the KKK I'd better not.