The question, however, is not if you want it, but if it should be disallowed. Being disallowed under 'sex is not as good' is really not what I define as an acceptable stance, given that you are under no obligation to engage in it and that it would disallow others what they would prefer.
(I tend to write too much, but there is one caveat in this - the roman influence could be natural selection, in a social sense, playing out. In this case, polygamy would be inferior, socially. Having said that, generally poly is better genetically. Way too involved to really know!)
But debate tricks aside, the point I wanted to make in that post was that this applies to marriage too - humans are 'equal' between both systems. An additional outlet would take mid-range cheating in monogamy and transfer it into marriage (of multiple mates).
Can I make it tangible? I see it kind of like the way it operates in some parts of asia now - 'duty' wife and distant mistress, or more like the Thai concepts of 1st wife/2nd wife. And it's illegal there, but... it's so standard that there is an entire market dedicated to hosting these women.