>>1083516Humans as animals typically mate with one person and mate for life. Monogamy is not something taught.
As long as humans have lived and been able to, we have married, and we have generally only married one partner. Poly-amorous relationships and open relationships rarely work for those involved because it is at a primary level contrary to our basic human nature.
If poly-amorous marriages were legal, you might have a few wealthy men marrying large numbers of women and siring many progeny from them, but what is to stop them from doing it now? They just cannot marry the women after all, however they can sleep with as many as they want. What stops them is their primary mate does not want to share, and they do not want another long term mate, or don't want to lose the one they have.
Essentially you are speaking out of your ass with no clue what you are talking about. Marriage and one partner relationships are older than our species, Australopithecus for example mated exclusively with one partner as well. For several generations as a species we have been exclusively one partner creatures.
As for incest, the only moral dynamic that matters outside of the potential for abuse that comes from many incestuous relationships is the risk of progeny and the genetic issues that spring forth from baring children to a close relative. Since between two women that is impossible, as long as it isn't done in an abusive manner, why the fuck would anybody give a fuck about two sisters having sex? It harms nobody as long as there is no abuse, and if it makes them happy, its not like, as two gays, they could realistically biologically reproduce(not that science hasn't gotten around that, though I imagine the cost and the fact that most scientists likely are not entirely willing to create sperm from stem cells to make incest babies anyway).