I also find Heinlein preachy. He has a definite POV as to how things should be, and it really comes through after you've read all of Heinlein, something I did one summer when I was 19 or 20.
He's definitely anti-religious, or at least anti-Christian. Christianity takes a hit in a lot of Heinlein, starting with the stuff from Revolt in 2100--the Nehemiah Scudder stuff. I seem to recall that Islam gets more of a pass. Correct me if I'm wrong. I'm 36 now, so it's been awhile since I've read all of Heinlein.
He also has a rather radical attitude towards sexuality and sex/love relations. I'm not sure where this comes from, and there was a time when I thought that his POV was more enlightened than the prevailing one. But the more I read, the more I find that evolutionary psychology says "no way." There are a lot of human behaviors that may make no rational sense to some people, but they are there because they were necessary for survival. Given the nature of life in the prehistoric period, monogamy and the instinct towards monogamy were survival mechanisms, helping to insure the survival of the next generation. None of that matters in 2005, but our genes are slow to catch up.
This seems to be Heinlein's biggest problem--there is a branch of science, one which is currently moving ahead in leaps and bounds, that he never takes into account. I suppose that's understandable, given the hostility most liberal-minded people had towards certain branches of genetics in the wake of eugenics programs in the early twentieth century, culminating in Uncle Adolf. But it still makes his work terribly dated at times, and still constitutes an ideological blindspot, historically warranted or not.
As for my initial reaction to MIHM, I had forgotten the narrative style, with Mannie's dialect. I think that's a pretty bold move for Heinlein to make as a writer. Granted, by 1966 he was already a Grand Master, but it's still a bold move, and it could have backfired in a big way.
In terms of content, I'm still working on that. Yes, it's that professorial streak coming out. I fudged it in class today and did the "small group study questions" thing, (1) to avoid having to lecture and (2) to try and wean their overreliance on me to tell them "what it means." They've been a bit too passive lately, so I'm trying to break them of it.
More later, or early tomorrow. I'll also hit the boxes in the garage and find some primary sources. Not so much because I feel the need to prove my point, but because there's some interesting shit in there.