Fiction is in part a imitation of reality. If someone is given impunity because of their status they'll commit injustice. Nobles and royalty had impunity toward commoners so they committed unjust acts towards them.
In real life there are often times where people both benefit from a relationship but after a few years they change and no longer benefit from being with each other. No one has to have failed for a relationship to not work out.
This nonsense is plainly disproved by the number of religious wars fought. Just being in different christian sects was used as grounds for war let alone following a different religion all together.
The Nazis, a non egalitarian group, were responsible for WWII, not egalitarians. Far right...
Gay incest has no possibility for children being born with genetic defects so it's better than heterosexual incest on that account.
Also don't be a homophobe this is the 21st century.