I haven't watched the video yet, so if he's pushing from a religious point of view then I don't defend that[*]. But note that the concept of multiple marriage doesn't equal "oppressive, ancient and backwards" - especially considering that the tradition has long been exclusive marriages, I would argue the opposite.
Multiple marriage - or more generally multiple relationships (polyamory) does solve the problem of "well I like you, but I've already got a boyfriend", and in places where one gender outnumbers the other, it's never nice to see several boys after one girl (or vice versa), and all the relationship-stealing, unfaithfulness and backstabbing that goes with that.
But ultimately I don't think you can posit it as a solution, people should be free to do what they prefer. Well, except that in our oppressive, ancient and backwards society, multiple marriages are illegal, so we aren't free to choose here.
([*] Though I still don't get it why religion is viewed as bad when it comes to multiple marriages, but the complete opposite is true when it comes to monogamy - i.e., so many people think it's intrinsically a religious thing and that that's good...; imo, religious marriage is ancient and backwards full stop.)
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