The longer the struggle over legalizing same-sex marriage goes on, the more ridiculous the arguments of those who oppose it will become. This is because they have no sound arguments, so they keep searching, searching, searching for something with which to clothe their increasingly chilly stand in trying to protect what is a heterosexual privilege.
Roger Scruton and Phillip Blond might just as well be telling you that people who are forbidden by law from entering a certain profession should not be seeking to change that law since to do so would be to 'erase difference, destroy institutional distinction and remove proper and plural provision for different groups'. That is their argument: that equality shouldn't mean the erasure of difference. The inconvenient circumstance that the difference at issue here is one which gay people experience as a disadvantage, and this for the very good reason that there is no justification that has yet been found to explain its necessity, doesn't seem to unsettle the thinking of these two friends of diversity.
And should you ask why, according to them, marriage must be preserved as a distinctively heterosexual union, the only answer you will find is this one: 'recognition of their [men and women's] role and unique responsibility in creating and nurturing children'. Scruton and Blond fail to explain why, then, they don't regard creating children as a legal obligation of marriage; or why, to put it differently, non-procreation should disqualify only same-sex couples.