It might help your understanding if you don't consider monogamy-only teachings as "normal". Basically, through most of history, most societies have accepted men having more than one woman. It's just the natural state of mankind - men die off in dangerous jobs (e.g. hunting) and wars, but protect the women from these dangers, leaving a surplus of women, who could not marry if they did not share. Obviously even in that situation many if not most men would be monogamous, but acceptance of polygamy for some men is the natural state of mankind.
Then, some religions layer additional rules on top of that natural state.
The Bible never does so. However, the Roman Catholic church limited most men to one wife, and priests to no wives. Initially however they turned a blind eye to concubinage (for much of the early history of the church many priests had concubines). So in some cases men could have as many women as they liked, just call no more than one a "wife".
The Quran sort-of limits men to four wives (it says you can have one, two, three, or four, and scholars have traditionally interpreted that as being an exhaustive list of what is allowed even though it doesn't actually say more than four is banned). But again Islam has traditionally turned a blind eye to concubinage, so once again men could in practice have as many women as they liked, just couldn't call more than four "wives".
In both cases these are legalistic rules invented by religious authorities and imposed on the people. They then find proof-texts in the scripture the people already accept and say it must be interpreted in this specific way, which agrees with the new legalistic rule. So Christian preachers will say "Adam had only one wife, and by this God means that His standard is for men to have only one wife and anything else is not ideal and therefore sin". While Muslim Imams will say "the Quran says men can have 1,2,3 or 4 wives, and this means that this is God's standard and anything else is not ideal and therefore sin". In both cases the new pronouncement is obviously contradicted by the fact that the most respected historical figures of that faith had more than the new limit of wives (Abraham, Jacob, David etc for Christians, and Mohammad's 13 wives for Muslims). But the religious authorities claim that God overlooked it in their specific case or at that time for some invented reason (the excuses differ), so you should ignore that.
These are religious weasel-words to justify man-invented rules. They are not the original words of God nor the natural state of mankind.
This is all understandable as there are reasons why religious and political leaders would want to limit the size of families, but that's another issue.
