Can a Catholic man marry more than one woman (at the same time) or is that a sin? Why?
No.
At one time it wasn’t a sin.

The conclusion from the Solomon story is that for the sake of love of a mortal, a tacit approval of one of his sins, and it should never be forgotten,
of the mortal ‘end severing’ variety, was accepted as a class of permitted behavior.
[Paradoxically, Eve was also innocent and favored prior to her sin, just as in Solomon’s case. But has we know there was no looking aside in her case. So we wonder if the need to stigmatize man played in as a factor.]
1/ So we can infer that sin can metamorph to a good activity.
2/ Something can be sinful now, but not later and vice versa.
Since only good can emanate from God, one needs to explain the
official acceptance of evil by Him. By this I mean our sins are never tacitly approved, but this one was(so was divorce), that is the difference. This makes the sin only a preference by man that is not welcomed but accepted by Him, and of course there is no such category. Either our acts offend God calling for justice, or they please God rewarding life; there is no accepted grey zones. Grey zones have been covered in the direction, '
One cannot serve two masters.’
As an aside and related, we need to study the consequences of the official acceptance of evil. For instance, a King who has 50 wives, 200 children and never could ever hope to have an unsatiated libido from countless nights of bed hopping, and,
isn’t favored by God, would usually find himself in a conflict of interest if he were to give advice and portray himself has the epitomy of sexual virtue and restraint. This effect being sin compounding as we would classify it, and in a religious context as he intended, heretical in nature. Instead he becomes the official Dr. Phil of Antiquity; and since he has God’s backing, man needs to sit back tongue in cheek and swallow this advice whole.
Not only does he need to digest the advice, but do so on an upset stomach, the result of the revulsion against a defence mechanism that was built in by the Creator himself, and tells our conscience that an exception is being made to accommodate a life desired, that if it were any of the rest of us, surely be food for the discernment process, any error having the potential to exinction. So the claxton of our conscience will wail uselessly, waiting for the creator to reset it.
You figure.

(Not that I’ve got my sites on Mormonism of course, one is enuff! ).
AndyF
.