When the man threw his skirt over the woman as to hide the act, that was considered an act of marriage.....a sort of confirmation as to their commitment to each other.
There were no licenses or permits to be applied for. You have heard of course of "shotgun weddings"......if a man made a woman pregnant, he owned it and had to care for the family as a whole. Perhaps they had "stone weddings" in the day. Hormones rage in the young and that was no different then than now, but then they did not have the government to supply an unwed mother with the needs of shelter, food and clothing.
Young men and women should be more thoughtful as to consequences, but in the heat of battle, they lose sight of what is down the road. My wife and I were fortunate enough to have children when we were ready, if there is such a thing. I was 17 and she was 16 when we started fooling around, but I did not father a child until I was 25. If by chance I would have become a father at 18 I would have owned up to it. It would have been difficult but it can be done and should be the law of the land, but it is not, but then Isaiah told us that in these times "Everything that is right shall be called wrong, and everything that is wrong shall be called right". And the beat goes on..............