Arguments about Adam and Eve are all well and good, but I don’t think they’ll solve the OP’s problem.
You might instead point out that every society has forms for marriage of some kind, whether monogamous, polygamous, polyandrous (sp? one woman, several men), etc. They also have some very stiff penalties for adultery, however they define it.
What every primitive society understood was that it took some teamwork to raise kids successfully, and it behooved them to put some level of stability and responsibility on the pair bond. Contrary to the snotty brat I had in 9th grade CCD last year, “shacking up” is NOT the “natural” norm or even widely accepted until very recently.
So, yes, many generations did not stand up in a church and say, “I do,” but they had something that tried to approximate marriage, which only proves the importance of the sacrament! It’s in our very nature to seek marriage. In fact, I would argue that some of those societies did better at marriage than the Western world is doing today with 50% and worse divorce rates.