The incest taboo does not have to do mainly with inbreeding. This is where we disagree. The core of the incest taboo is in blood relations and kinship. While there are a variety of ways to define kinship among societies, the most important forms of kinship are universally framed in terms of biological descent (blood). This is what the incest taboo is opposing, sexual relations among closely related kin.
There are cultural variations as to who is included in this taboo and to how violations of the taboo should be dealt with, but the taboo is present in one form or another everywhere.
It is not biological degeneration that drives people to put mechanisms in place to deal with incest, as societies were not always aware of the negative effects on genetic material, but they still had rules regarding incest. So while inbreeding may now be part of people's reasoning, the incest taboo was already present before this. Furthermore, there have been incest taboos found among societies who had no way of knowing the biological effects (the anthropologist in me wants to come out)
It's really not from this side

It's a matter of how you look at the situation. It's is not the fact that two men have sex that causes people to morally object to this thread's theme, as evident in several homosexuals members here opposing to sibling sex, but (obviously) not to gay sex.
It's the sibling part that makes it morally objectionable to people. And I'm not saying everyone should think this way, but societal rules for incest are of a different nature than those relating to two non-related males having sex.
You can see it as picking and choosing which of society's rules to follow and you can make that argument and have it make sense for sure, but it'd still not be correct in my opinion. Certain societal rules are stronger than others, some are more universal than others, some change fluidly while others stay rigid. It's hard to explain why some rules are deemed right and some are no longer deemed correct. But the incest rule will be here for a long long time I predict as blood relations and kinship relations based on blood are integral to human society.
Society changes, it adjusts. And while some societies may still oppose to two males having sex, most have adjusted. Some a little, some a lot, some not at all. But societal rules against two unrelated males having sex are not nearly as strong as societal rules against two closely related people having sex. Basically... Incest trumps gay. I'm not making the rules, but this is what it is.
I'm not objecting to your opinion either btw... I get where you're coming from, at least I think I do

Could you explain to me though how you see a brother-sister sexual relation as different from two brothers having sex. Because siblings are siblings, which is why, I think, members on here are saying it's wrong. Is it because brother-sister relations can produce offspring?