QUESTION: During the ancient times of Ramayan, there were singular marriage partners. By the time of Mahabharat, multiple partners were the norm like Draupadi, all Pandavas having wives apart from Draupadi, Dhritrashtra having kids from Gandhari and a maid, Vyāsa furthering the Kuru dynasty from princesses and maids, Shantanu having Satyavati and Ganga as wives, etc. So why in kaliyuga singular partners is becoming a norm? Shouldn’t it be moving towards multiplicity?
Your information is wrong.
In Rāma’s time (Treta Yuga) it was very, very unusual to have only one marriage partner. Therefore Rāma is renowned for “eka-patnī vrata” — His “vow” of having only one wife. Rāma himself, for example, had three mothers, because his father had three wives.
Prior to Rāma’s time (Satya Yuga) there was no marriage at all! People were naturally virtuous, responsible and loving, so there was no need for laws and rules. Fathers would automatically be caring, loving, supportive and protective of their children. It was during Pṛthu’s time that many, many things changed in society. Government, laws, courts, and so on were set up, because the natural virtue in humans began to decline. Marriage was then instituted as one of those laws, but it was not limited to one man and woman. Some women had many husbands, most men had many wives. (Divorce was also allowed, even in the recent lawbooks like Manu-smṛti). This type of marriage continued into the time of Mahābharata (Dvāpara Yuga), and into the very early portions of the modern Aeon (Kali-Yuga).
But in this Aeon exclusively monotheistic and dualistic religions arise, and their adoration of singularity and disdain for normal pleasures made, for many many centuries, monogamous marriage the only lawful context for sexual intercourse and child-rearing.
These religions seem to be losing their hold over the past century or two. That will probably continue, and society will probably fall into an entirely irreligious state (though there is hope that for some/sometime there may be a return to the natural, healthy ancient religious outlooks). In that lawless state polyamory again becomes the norm, and eventually marriage will be a very outdated, abandoned thing that disappears. Thus, it will look a lot like Satya Yuga, on paper, but in practice it will be completely opposite. In Satya Yuga people had no rules and laws about love because they were naturally, inherently virtuous. In Kali Yuga people will have no rules and laws about “love” because they are naturally immoral and cannot stand to be responsible and dutiful.
As Kali-Yuga progresses we will probably find more and more semi-spiritual people justifying their lusts by reference to the polyamory and anarchy of previous Aeons, but in truth most all of the polyamory and anarchy in Kali-yuga will be exactly the opposite of these ancient times, because in this Aeon people are raised to be very selfish and self-serving, and it is very difficult to remove those patterns from our psyche.