A true love marriage of innocent hearts can make it much easier to forsake all others.
Nevertheless we must consider our fallen natures. It is inevitable for most people to encounter temptation. And it polygamy has been allowed by God in the past.
But in the beginning it was not so. Man made Adam and Eve: each for the other exclusively.
You should point out that the quote "from the beginning it was not so." was from Our Lord.
It's true that polygamy was practiced in the past by the Patriarchs.
It's also true that polygamy isn't opposed to human nature, as compared with, say, polyandry, promiscuity, or "open marriage". The latter either make no sense, or cause nothing but problems.
The Mormons seem to make polygamy work -- at the expense of the females, of course. They consider themselves one big family, and the co-wives consider each other "sisters". They put on a smiling face for the television camera, but I'm sure female jealousy is there nonetheless.
As Christ said, man was created for one woman from the beginning. Polygamy was permitted for a time, but now that we have the New Law -- the higher law, instituted by Christ, marriage has been restored to its pristine state, thanks to the New Creation effected by Christ on the Cross.
That is why Our Lord's side was opened (like when Adam gave a rib for God to create Eve), he submitted His Will to the Father (to atone for Adam doing his own will) -- Our Lord's whole work on earth mirrored, and perfected, the first Creation.