Should the government legalise gay-marriage?
The root of this question is grounded in: “What is the role of government in marriage?”
Abstracting out the ceremony, at the end of the day; marriage is nothing more than a contract.
Thus government should have no more involvement than they do in officiating any other contract.
They must ensure that all parties involved in the contract are free from coercion and fully-aware of the terms, before legalising the contract.
The problem with marriage is that the government treats it as more than a contract, because of its religious roots. The specific ceremonies applied to marriage-contracts are completely irrelevant.
In conclusion: Gay marriage shouldn’t be legalised, instead normal marriage should be reverted “back” into a contract.