Is the default in marriage polygyny and the exception monogamy, and is one who is capable of polygyny, justness, and providing for multiple wives sinful if he does not do so, especially in our time?

The statement that one who avoids polygyny is sinful is incorrect. The fundamental principle regarding marriage is permissibility. There is no evidence to suggest that polygyny is the default and limiting oneself to one wife is an exception. Some scholars have explicitly stated the desirability of limiting oneself to one wife when there is no need [for more].