It is implicit in your argument that special status for traditional marriage is improper and/or unjust. But how does adding homosexual relationships change that? Are those the only sorts of stable relationships that benefit society? The current Prime minister of New Zealand grew up in a solo parent environment. Clearly that was positive for society, so shouldn't that sort of stable beneficial relationship receive protection too? And what about communes, or extended families? Those sorts of stable relationships can be very positive for society. Where is the special status for them? The whole argument seems to be that adding another group of privileged will erase the injustice. it is self-evident that this is only true if you happen to be in one of the privileged groups. Whereas erasure of the special status that must have mattered once but apparently no longer does, certainly removes the injustice completely. Somebody needs to explain why it is fairer to shift an injustice than to remove it.