Heinlein is known for sprinkling his books with less-traditional sexual dynamics shown positively (e.g. polyandry in "The Moon Is a Harsh Mistress", general sexual freedom in "Stranger in a Strange Land"). But that was quite possibly largely driven by the demand of storytelling (male to female ratio on the Moon) and not necessarily his personal views.
Did he ever express an opinion on the topic in actual real life society?
Please note that I'm excluding "normal" homosexuality, for which I already know the very-libertarian-attitude answer; to quote RAH's letter:
But moral repugnance? So far as I can see, the behavior of homos is harmless and none of my business. I habitually smoke cigarettes — a habit at least twice as “dirty” and ten times as harmful — or perhaps infinitely more harmful, since cigarettes are probably harmful and homo play probably is not.
The only thing shocking to me about homosexuality is the shocking way in which we persecute these eccentrics.