Marriage is based on a man and a woman who rear their own children

It seems to me that this issue is being totally misunderstood - the UN in its Universal Declaration of Human Rights of Dec 10, 1948 [ Art 16 ] was not reflecting a Catholic or indeed any religious dogma when it defined marriage - it rejected as unjust discrimination merely any state limits on the right to marry based on religion or race or ethnicity, while the European Convention on Human Rights of 1950 and the UDHR, like both the US and Irish Constitutions, have no explicit or even implicit right to homosexual marriage and the vast majority of States do not provide it.
The core issue is simple - every child has the inherent and fundamental right to both know, and be reared by, their own natural parents, while every father and mother has a strict ( and non-transferable) duty to rear any children they have conceived together.