Searched for awhile for defunct gay civilizations... this is about all I could find:
Very Bad News - The New York Review of Books
Oddly enough, none of the defunct civilizations mentioned were homosexual in nature...
"The dangers of catastrophe are growing. One reason is the rise of apocalyptic terrorism. Another... is the breakneck pace of scientific and technological advance.... The cost of dangerous technologies, such as those of nuclear and biological warfare, and the level of skill required to employ them are falling, which is placing more of the technologies within reach of small nations, terrorist gangs, and even individual psychopaths. Yet, great as it is, the challenge of managing the catastrophic risks is receiving less attention than is lavished on social issues of far less intrinsic significance, such as race relations, whether homosexual marriage should be permitted, the size of the federal deficit, drug addiction, and child pornography. Not that these are trivial issues. But they do not involve events of potential extinction or the modestly less cataclysmic variants of those events."
hmm...
that would seem to imply that our danger of killing ourselves off due to social/moral issues is pretty minimal in comparison to us simply breeding/eating/consuming/bombing ourselves out of house and home.
Seeing as most of the defunt civilizations in that article died off from deforestation, land use, starvation, natural disaster and over-population: maybe exclusive homosexuals are doing us a favor in not reproducing?