So you think there wasn't a single rational justification for the age-old, virtually UNIVERSAL definition of marriage as one man, one woman?  What hubris.

Your citation of Article 10 of the Declaration of Human Rights does nothing to support you cause.  The right to marry is FUNDAMENTAL in the United States as well, and yet it woud have been though laughable, until recently, to say that this fundamental right to marry included the right to marry someone of the same sex.  Why?  Because "marriage," by definition, meant one man, one woman (although in some instances, polygamy), but never man-man or woman-woman.    Note, by the way, that the Declaration, Article 10, says "without any limitation due to race, nationality or religion."  It does not forbid a prohibition on grounds of sex, meaning man marrying man or woman marrying woman.  Indeed, such a prohibition would have been though absurd when the Declaration was passed in 1948, because the very idea of such "marriage" was absurd.