First comes Love.
Then comes Marriage.
Then the Baby and the Baby Carriage.
Love needs to be selfless and confirmed to survive the rigors of life. Right now, the Boyfriend can just leave on a whim, leaving his first born without a Father; the wife becoming a single mother.
Marriage in front of witnesses gives a new family (Husband and Wife) a foundation to build on. Do it before witnesses and before God, even more so.
With selfless Love and a good marriage foundation, the nest is created; a home secured and lovingly lined for the hoped for arrival.
Get the little song wrong, poverty, despair and desolation are probable.
Marriage is different than merely shacking up. You give your word to the other person, the assembled company, and to God.
A person is only worth what their word is worth. Your word is the only thing you truly own. This is why I have no regard for a politician or supposed leader who cheats. In doing so they are telling you that their word is worthless. And so are they.
Then comes Marriage.
I heard an interesting take from a preacher who says it's the reverse. Marriage is the foundation for Love, not the reverse.
First you Marry, during which ceremony you promise to Love the other person till death you you part.
That means you will yourself to love that person, every day. If you don't or can't love that person one day, you try again the next day, till death do you part.
The mistake people (especially women) make is that they think that Love is the foundation for Marriage. And if they no longer feelz the love, then "the marriage is over," so they get a divorce.