[ViscountPenguin]

Living in a country where atheism is the most common religion amoung my age group, I'm not sure that I've ever seen marriage as a religious arrangement.

I think for most people it's more of a costly signal of commitment. Although some people prefer a civil partnership instead, to codify a defacto relationship.

[coffeebeqn]

It’s not necessarily costly. You can have a cheap wedding and in the US for example you get access to “married filing jointly” tax treatment which can lower your taxes and increase your retirement savings in many cases

[ViscountPenguin]

I can't speak to the us, but where I live there are no tax or legal benefits to being married vs being a defacto couple (apart from it being slightly easier to prove if it ever goes to court)