I think your narrative of hunter-gatherer societies might be off. First, why 50,000 years ago? Agriculture is usually said to have emerged around 10,000 years ago, although there is evidence that it might have happened earlier. Our current modality of marriage and monogamy is a by product of property ownership, which is a by product of settled societies, which is a result of agriculture. In hunter gatherer societies, which are usually nomadic, you wouldn't have any of that. They were far more egalitarian in class and gender, with clear roles for men and women, but those roles were equally important. They also tended to be sexually poly-amorous. It's easy to assume these societies were male dominated since men are naturally stronger, but the role of women were also extremely important. A lot of them if anything, were matriarchal. These societies also had higher infant mortality rates and since the survival of a tribe rested on having healthy babies, female fertility was emphasized greatly, which you can see in their art.
In our modern society, people feel the need to get married to stave off loneliness. We don't have tribes anymore who have our backs until we die so we make arbitrary partnerships and try to force them to last to combat our loneliness. From a practicality stand point, I think marriage is only useful if you intend to have children. Even if you believe in the futile notion of life-long monogamy, you can still be with your partner without a legal agreement to bind you together. But a lot of legal issues would pop up with kids born out of wedlock.