The problem with davem's analogy is that Middle Earth is not a house. It's a history of several civilizations spanning thousands of years in varying degrees of detail; there are large parts where we are not explicitly told exactly what happened and have to presume what makes most sense.
It's as if the house had hidden rooms which nobody had ever explored; the rest of the house may have been red and blue, but those rooms may well have been green.
The explicit inclusion of heterosexuality and asexuality does not imply the exclusion of homosexuality, just as the explicit inclusion of mammals and birds does not imply the exclusion of bacteria.
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