I was a huge fan of "Stranger in a Strange Land" as a teenager. And, even though I'd probably dislike it if I read it today for the first time, it remains a favorite because of the emotional attachment I have to it.
For those of you that don't like "Stranger," I'd suggest you give Heinlein one more chance and read "The Moon Is a Harsh Mistress." In it, he includes line marriages. Even if you hated "Stranger," you might enjoy this book, as it's very different.
And, to satisfy my own curiosity: Can anyone suggest other fiction that features polyamorous relationships? Either sci-fi or not.