History is a difficult subject. Even from the purāṇas we only have bits and pieces of what life (including sexual and marital norms) was really like in previous yugas.
The Vedas never say “one size fits all.” They never recommend to apply moral principles to everyone in the same manner. The Vedas recognize at least four different basic personality types (thinker, leader, business, and employee) and each one of those four goes through four stages in life (student, family, retirement, and spiritual dedication), and the moral principles apply differently to each type of person in each phase of life. The sexual-marital norms that apply to a thinker in the stage of spiritual dedication, for example, are extremely different than those that apply to an employee in the stage of family development.
On top of this the principles evolve with historical development and are markedly different in the four ages. The basic norms in the four ages seems to have been basically like this:
In Satya Yuga, there was no need for regulation of how men and women interacted. Humans were naturally inclined towards self-improvement and spiritual evolution, and naturally approached relationships in a healthy, responsible and loving manner.
In Treta yuga there arose conventions we now call “marriage” but these “marriages” are not like what he have today. They are more like “consensual relationships,” just with a bit more formatting and structure than what was happening in Satya Yuga. Polygamy as well as polyandry were normal during this yuga.
In Dvāpara yuga polygamy survived, but polyandry mostly disappeared (with the notable exception of Draupadī very late in Dvāpara yuga) – probably because female instinct is less prone towards multiplicity, and also because of the practical genetics of one male being able to impregnate multiple females, but not visa versa. Marriage in Dvāpara yuga became more formal. The less formal versions were depreciated and disappeared, and what they mainly considered “marriage” become similar to our modern conception of it – contracts, vows, and promises made with gods and sacred beings as witnesses. This shows the increasing need for strong regulation of how people naturally behave.
Now, in Kali yuga, our instinct is exactly opposite of Satya-yuga. We are naturally very selfish and irresponsible. So a huge amount of regulation is required in the matter of sex, to prevent a great deal of victimization (mainly of women) and neglect (mainly of children). So, during the initial millennia of this age polygamy depreciates and soon disappears. Stress goes very strongly to monogamous / mono-amorous ideals.
This, however, is extremely difficult to enforce, because (A) it is quite against the basic nature of sex and pleasure and (B) the very nature of kali-yuga is not hospitable to strict rules. So, what we are seeing now, just a few millennia into the yuga, is that marriage itself is disappearing away and we are reverting to an animalistic polyamory – quite opposite of the beautiful polyamory of Satya-yuga. In the emerging Kali-yuga version of polyamory everyone exploits everyone else as much as possible, to “get away with” as much as they can, and “get what they need” from their relationships.
This is why kali-yuga is depicted as the cesspool of dharma (morals).