I don't see what gay marriage will do in terms of added value, other than appease the equal rights of one group in defiance of another group.
How are the rights of people opposed to gay marriage being infringed by allowing it? Rights have physical outcomes, which is why gay people being allowed to marry would grant them rights.
Having an opinion that you don't like something, doesn't mean that it is your 'right' to have it banned/not made legal.
I oppose fox hunting but would never argue that if it were re-introduced my rights were being infringed (although I would argue the fox's were).
Polygamy rights are ignored, or like you point out, cousins can't marry.
In the UK they can, thanks to good ol Henry the 8th (who also changed the definition of marriage to include re-marriage after divorce which was unheard of an not recognized as 'marriage' before but he changed attitudes and now we accept it).
Equally allowing marriage for these groups adds nothing to the value of marriage as an institution so why bother?
Dunno, ask Henry 8th why is legalised cousin marriage and why all subsequent governments have not reversed it. Or a better question would if cousin marriage has been legal in the UK for nearly 400 years, why hasn't British society exploded?
Personally I see marriage failing at the same point Christianity falls out of sight
You do realise that marriage and Christianity are mutally exclusive things don't you? You do understand that the only reason Christian Churches can hold marriage ceremonies is because they are licensed to do so by the state.
I'll say it again, marriages are legal entities "owned" by the state, the church has no more claim to own or run them than my local garage does.