If you want a reasoned argument against legalizing same-sex marriage, er... forget it. You won't find it. As I've taken the trouble to document in earlier posts, what purports to be argument is of the flimsiest: marriage means a union between a man and a woman, or this is the kind of union hallowed by tradition, or marriage is for procreative purposes, and such; with the facts that the meaning of words can change, and that traditions can be altered when there's no (longer) justification for them, and that many marriages are without issue, seemingly overlooked.
The new Archbishop of Canterbury is reported to be going to say that 'marriage is between a man and a woman, and always has been'. No change on that front then.
One should hardly ever abandon attempts at reasoned argument; but in so far as reasoned argument sometimes doesn't work and people continue to appeal to continuity with the past even when they're short of ways of justifying the particular past under discussion, let me remind opponents of same-sex marriage of a couple of other cases in which adult human beings were unfree to marry whomever they wanted to (the other only consenting) and as they pleased. Ante-bellum slaves in the US could not marry legally without the permission of their owners. Under the Nuremberg laws in Hitler's Germany, Jews were prohibited from marrying persons of 'German or related blood'.
These were vile 'traditions' which opponents of gay marriage today would, doubtless, not want to invoke, and rightly not. Just one of the reasons they were vile is that mature adult couples should be free to marry one another, both parties consenting, and not subject to constraint in this by interfering others.