Hmm, here’s my two cents.
I’d say the argument that: “since marriage has changed throughout different cultures and times, we can legitimately change it now” is problematic for a couple of reasons.
Firstly, marriage has not changed in the quality that is at issue in the same-sex “marriage” debate, namely, never has a society at any time recognised widespread same-sex marriage as a norm, equal or even comparable to heterosexual marriage. The exceptions are very specific and often not anything like the same thing, for instance, involving some form of transgenderism (physical, social and/or spiritual).
Secondly, arguments from history aren’t necessarily legitimate. We don’t base our understanding of human rights on the Ancient Babylonians or even the Ancient Romans. Rather, we have a concept of what it means to be human, and, reflecting on (definitely not avoiding) history, we develop a better understanding. This is the way we should approach marriage too.
Thirdly, the argument may be pointless. Surely our aim as a society is to understand why we have the institution of marriage. The principal socially and politically legitimate reason for having this public institution is to join men and women in a small society for the purposes of child bearing and rearing. Since same-sex “marriage” is by nature infertile, one has to wonder why we even bother with it.