A conservative religious site quotes a Russian Orthodox Metropolitan on one of the most central reasons why conservative religious people are opposed to same-sex marriage:
Metropolitan Hilarion Alfeyev, the public face of the Russian Orthodox Church, says the continued relevance and existence of the WCC depends on how effectively it addresses challenges facing Christians throughout the world. He identifies two major challenges: militant secularism and radical Islamism. These challenges, he says, threaten Christian civilization and the essential nature of the human person. He says:
Countries that offer legal recognition to homosexual couples are “…taking a serious step towards the destruction of the very concept of marriage and family.” Traditional gender roles with the mother as “the guardian of the domestic hearth” and the father as the “educator of his children in being social responsible” are being undermined. He condemned politicians who endorse same-sex marriage and legalized abortion as “pronouncing upon their peoples a death sentence” as their policies lead to an inevitable demographic crisis.
There you have it, in a nutshell, and this is how the treatment of same-sex marriage links to the core of feminism. How can we pick the "guardian of the domestic hearth" if not by genitals???
Though that idea of the father as the "educator of his children in being social responsible" is most likely a euphemism for the father being the head of the household, the sole breadwinner of the family and the one who is expected to dole out punishments.
All this also links to the crudest type of evolutionary psychology (the kind I usually denote by Evolutionary Psychology), which is equally based on the "essential nature of the human person."
Except that in both thought patterns (religions?) women are stipulated to be innately more suited to guarding the hearth than men. It's like that drawing of mine where "human beings" is a main category and the category "women" dangles below it with no comparable category for "men."
What's very sad for the conservative religious insistence of women's proper role (in the kitchen, at home) is that the traditional gender roles are economically very difficult to sustain. For example, a British study recently found that:
Households with a lone breadwinner, traditionally consisting of a working man and a stay-at-home mother, are the biggest group living in poverty in Britain – according to new research.
The work, funded by the Joseph Rowntree Foundation, shows that a third of the 1.3 million families with children living in poverty are "single breadwinner couple families". There are half that number of two-earner families living in poverty.
This makes logical sense, though I have not read the study itself.