I've also heard that widespread acceptance of gays is one of the first steps to bringing your nation/empire to its last days. Most of the simple theories don't seem to pass the smell test.
Possibly the only compelling reasoning I've heard that can be summed up in a few sentences is that, when an empire becomes successful, its citizens' needs are met more easily and life, as a result, becomes less difficult. In absence of significant hardship on the home front, major sectors of the population eventually grow complacent and do less and less to prevent the corrupt from rotting out their government and economy from its core. A sobering thought in that, if there's any truth to it, every society that experiences any great measure of economic success is doomed.
Once the citizen's needs are met more easily their primary occupation become pursuit of pleasure. Their initiative is sapped, their will is bent towards feeling good above survival.