What are the best arguments you have in favor of marriage?
Societal pressure if you have kids. Not an issue in some areas.
In many countries, it's required of older folks to obtain legal residency. Besides that, pointless unless your religion proscribes.
There are enormous legal and tax benefits to be gained from marriage, but for the relationship side I don't think it is needed--a loving and caring partnership comes from something deeper than a marriage certificate.
In reading these comments, I think it would have been interesting to have also asked people to tell their status: never married, married, widowed, divorced (and was the divorce amicable). I can almost tell from the answers of some.
I am a Widower,lost wife of 27 years to lung cancer in Sept.2017, didn't marry until 43,so no kids. Why so late? 8 years in the Air Force,moving about every 6 months to a years time made it impossible to put down roots and raise a family with a Woman,seems I'd try to get a romance going and then new orders to move again,leaving a trail of broken hearts behind and bruising mine.
And then after separation,low paying jobs that kept me fed and housed but no extra to date with.Only when I got a better paying job was I able to do a wife hunt,so I ran an ad in a singles paper,my wife to be read it,called me up,we met,dated and married 6 months later.
No crabs anymore!
You're wildly optimistic!