You certainly don't need to get married if you don't wish to do so, and you can still live a fulfilled life with or without a partner. But marriage has never been simply a "religious arrangement". Yes, marriage has been much co-opted by religion, but non-religious people have always existed and have always married each other outside of any religious arrangement.
From a modern society's logistic point of view, marriage is not a religious arrangement. It is the recognization of a contract entered into by 2 parties which will dictate myriad events in the future, especially regarding property/child/estate/healthcare/etc disputes.
Neither me nor my wife are religious, but pooling our assets together and having children together means we are now intertwined to the level that third parties may need to intervene (in the event of a dispute or death or something) and it helps sort that out more easily.
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.
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
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)
I got together with my now wife at 20. We didn't get married until we needed to for a visa. Getting married didn't change anything, and we acknowledged that we were a couple for good well before then, had kids. It helps that the country we're in recognizes defacto relationships so there was no incentive.