Whether you spell it with an 's' or a 'z', I don't like the word. It is used so lazily and it starts with a rather dubious (French) assumption.
Let's assume that you favour the legalisation of something that is currently illegal. Shouldn't you be campaigning for its legalisation? Well, not really. You have surrendered before you start. You make it sound as though the default position should always be the status quo unless there is a good reason to change it. That's problematic. People will ask you to spell out all the possible consequences. Might there not be a few examples of things which go wrong? Can you guarantee that there won't be any? Obviously, you can't.
But the (French) Civil Law assumption that everything is illegal until licensed by the state is not an assumption that is suitable civilised discourse. In Common Law everything is legal unless specifically banned for publicly disclosed and (we would hope) wholly defensible reasons.
We have been through this with debates on privatisation. How, exactly, will the market provide service X? What will happen if a few people are left behind? Markets depend on the ingenuity of millions. No-one is smart enough to anticipate in advance all of the permutations a market will adopt. What will happen if the postal monopoly is abolished? Will the new system be perfect? Probably not. Is the current system perfect? Certainly not. Would a privately run system be better? Almost certainly so.
The same is true with repealing drug prohibition. If you are advocating some change it is always best to focus on the weaknesses of the system you oppose, not possible weaknesses in the system you wish to establish. This is particularly so when the policy you advocate is freedom. We don't know, we can't know, exactly what will happen if people are given a choice that they don't currently have.
Surely the default position is not the status quo. The default position is freedom. People who wish to restrict the freedom of others have the burden of proof. Explain now, but continue to explain as long as you wish to maintain the law why it needs to be maintained. You don't just get to win the argument once. You need to keep making the case, even after your policy has been adopted. Perhaps particularly then, when we can see the flaws in the policy.
Which brings us to gay marriage. Should it be legalised?
I am not aware of anywhere in any western country where it is currently illegal. What it is is unrecognised. Some US states used to make inter-racial marriage a crime. Mildred Jeter and Richard Perry Loving married, quite legally, in Washington DC. They then settled down to live in Virginia. They were prosecuted for their temerity in doing so and sent to prison for a year each. I do not know of any jurisdiction in the US where that would happen to gay people who had married in Massachusetts. Their marriage might not be recognised by most other states, but it would not be illegal.
So comparisons between the legalisation of inter-racial marriage and the campaign to recognise gay marriage are a bit of a stretch. But they all depend on the lazy use of this one word.