Previously the Alpha was defined. The wolf just below the alpha in the hierarchy is the Beta, and it is implied by this that he is very competent. Yet with wolves, only alphas mate. And in modern times the situation appears to be trending that way with humans also.
As mentioned above, the beta is competent, but not dominant. These are the kinds of men that civilization needs in abundance to thrive. Men who get things done; ones that do the work themselves. As for dominant men, civilization does not need that many of them; as they tend to jockey among themselves for dominance (with no useful work being done).
For the longest time civilizations cultivated and rewarded such men. This is how in large part they became civilizations. But in the present day, it appears that civilization believes itself wealthy enough that these men no longer need to be cultivated and rewarded. Part of that reward was an expectation that comparable level women available for marriage would be in the offing. Followed by a family to call his own. For the modern day beta men, marriage and children may eventually become available (often much later than in earlier generations), but the family would never be truly his. It is hers, to do with as she sees fit; so says the law (effectively).
Modern Beta men often spend their late teens and twenties developing job skills and building a small nest egg. Typically during this period, they have little success with women, as they do not generate the one thing in women that they seem to value above all else — tingles.
But a woman cannot live her life by tingles alone, so as the modern Beta man enters his thirties, he gets interest from women. But the interest is typically due to the woman’s need for sperm donation and future provisioning (including possibly children she brings into a marriage). Perhaps she temporarily convinces herself that she loves him, but she may not be able to maintain this fiction for long. Her resentment builds and after she bears her desired amount of children, he is pushed out the door. To continue paying for twenty years or more. This is a common trajectory for a modern beta man.
It was not always this way. My Dad was as beta as they come. Yet he married, had a loving wife, and a family to call his own. Women were raised differently then. And society had different expectations.
P.S. As always NAWALT applies.