1) there've been entire civilization based on incest.
Yeah, no. Segments of the ruling class do not equal the entire civilization. The whole point was that they were considered special (usually meaning some degree of divine) and beholden to different sets of rules than the hoi polloi.
Or were just trying to sidestep the usual dynastic headaches by "keeping it in the family" - ask the Spanish Hapsburgs how well that works out long term...
3) Ah ? Did you hear of marbled crayfishs ? Can live in about any sea whatsoever, numerous, giants, yet, all females clones (parthenogenesis). So much for diversity, huh.
Parthenogenesis is overwhelmingly effective for sheer reproduction; rather less so for adaptation which is kind of the major biological selling point of sexual reproduction. (A certain level of diversity is nevertheless sustained by multiple self-replicating "lineages" within the species and the usual random mutations though.) It works fine as long as the circumstances stay stable but the species runs a pretty high risk of being wholesale fucked if there's a major upset in its relevant ecological conditions.
Which would be why species relying solely on parthenogenesis are fairly rare.
Also completely different from the biological driver behind the universal prevalence of behavioral (cultural and psychological for us humans) aversion to consanguinity, which is the accumulation of harmful recessive traits.
The aforementioned Spanish Hapsburgs could tell you a lot about that, too, if they hadn't quite literally inbred themselves unto extinction (which set off a major general European war that ended in a bit of an upset of long-established balances of power, but I digress...).
Also, mountains gorillas.
Hardly the only endangered species with serious inbreeding issues due to severely depleted gene pool and fragmented populations. (Africa alone has only too many examples.) Not quite seeing the relevance...?
'Course, the genetic/biological angle is for obvious reasons irrelevant in the context of a homosexual relationship but eh.