Okay, the unnatural argument's a non-starter. It's also unnatural for us to have access to modern medicine and other technologies that help keep us alive. No one argues that we should all die at seventy because we're only allotted threescore and ten years (okay, fourscore "if by reason of strength). So clearly we're not too wed to the natural order. There is no evidence that the children of gay parents are any worse adjusted than the children of straight parents. We've seen homosexuality in animals, and even "same-sex" animal couples raising young. So a much better argument needs to be found than this.
Is it acceptable to Western Civilization? Getting more and more so every year. And once
Finally, there's the question of whether or not it's acceptable to Christians. This is somewhat more relevant in Britain than it would be in the US, since Britain is explicitly a Christian nation. And yet despite having a state religion, Britain remains a strongly multicultural society, with Christians, Atheists, Muslims, Jews, and Hindus all rubbing shoulders. The laws need to represent the entire society, not just one group (even if it is the largest group). And finally, when one considers the sheer number of people who support gay marriage, and the Christian majority in Britain, the conclusion must be drawn that, at least to some Christians, gay marriage is acceptable. Christianity is not a monolith. There is no single answer to "is this acceptable to Christians." To some Christians, it's perfectly acceptable. To others, it isn't. To some Christians, the legality of marriage by other faiths is wrong, or the marriage of different races. This does not obligate any other Christian to hold those beliefs.
Ultimately, the question comes down to whether or not gay marriage is acceptable to the people of Great Britain, Christian or otherwise. Whether or not someone thinks they should or not should have no bearing on anyone's conscience.
