I think afleitch means that some things are just morally neutral. Gay marriage proponents aren't arguing that gay marriage should be legal because it's moral, but simply because it isn't immoral, at least in an objective sense.
Objective by what standard? And how is that standard any less subjective than a traditional Old Testament based standard of morality? Objective is one of those buzzwords one really needs to be careful about using.
It’s entirely neutral. Given what we understand about human sexuality it has to be neutral. [...] Deep rooted same sex attraction is part of humanity. The same bonds between couples exist and have the same value.
Is your understanding correct? A Christian fundamentalist would likely argue that homosexuality is an imperfection introduced into man via the original sin of Adam. Pointing out that homosexuality exists in the animal kingdom would not shake the resolve of a true believer since in that world view, the taint of original sin caused by the fall of man affected the whole of creation. As for bonds, don't tell a chemist that a covalent and ionic bonds have the same value.
As a society, we're embarking upon a large scale experiment on the social impact of same-sex marriage and the fact that nothing major has gone wrong so far is a guarantee that it won't. After all, if one skydives without a parachute, nothing bad will happen in the first few seconds. Not that I foresee any problems resulting from same-sex marriage, and it will be at the very least difficult to perceive any that might result given that we have yet to fully understand the impact of the ready acceptance of divorce and remarriage upon society, a change for which potential problems are far easier to perceive in advance.