Hinduism predates Christianity, Judaism and Islam, and The Rig Vedas (the oldest Hindu scripts) refer to homosexuality as the 'third gender''or ''the third kind'', as a normal part of nature, thus it is not ''the morality of all human history'' nor ''the scripture of all human history''. Homosexuality has existed in humans for thousands of years, and it is also witnessed in many animals, so it is not ''the nature of all human history'' either.
It most definitely is YOUR morality, for if it was the morality of all human history, then people wouldn't disagree with you.
Two (adult) brothers marrying each other is illegal, but consensual incest, in various US states, isn't illegal; Ohio, New Jersey, and Rhode Island. As for two brothers marrying each other, the practice would be pointless; they are already related, and the secular function of marriage is to create legal familial ties that benefit the appropriated parties.
As for why it's morally unsound for two consenting male brothers to have a relationship, I can't think of any reason outside religion, law, or personl disgust for that. It's at present wrong because in the vast majority of people it produces revulsion (it does that in me, too) and of course because it's currently illegal in most places. But if there comes a time when most of humanity want to have incest (which there most likely won't, because we're biologically programmed to want to diversify our gene pools by finding partners outside our immediate family) then the social perception of incest will have changed.
Most of the time, what we find morally repulsive is subjective and a matter of consensus of opinion. A hundred years ago, a woman being a man's boss was probably pretty repulsive. People might have even considered it immoral, but there was no genuine basis for it being viewed that way other than that it broke tradition, custom and offended men.
Incest that does not lead to procreation, I can't really say is difinitively immoral, but certainly it is against our nature, our desire to seek partners outside family, and of course the idea of doing it revulses pretty much every human being.