[stonemetal]

>marriage is a religious institution, seems pretty clear.

No one is debating marriage the religious institution, you can go have whatever private marriages you want, and religions can still discriminate. What was being debated was the somewhat different, marriage the legal entity that effects inheritance, joint custody and all the other legal benefits that spouses get. They commonly occur together but they do't have to.

[cmdrfred]

Clearly, but why do married people get any benefit that people who are not don't have? Isn't that in and of itself discriminatory? Isn't the government defacto promoting region in general by giving tax breaks to people who choose to contract with each other in such a manner?

[stonemetal]

Marriage as a legal entity is a legal recognition of a social custom. While some prefer to establish that social custom with a religious ceremony, there is nothing inherently religious about it. For example Buddhism has no religious institution of marriage, yet countries that are primarily Buddhist still have marriage as a civil entity.