You can see why monogamy was so rare— it effectively ended his line of succession.

“Unlike almost all of his predecessors and successors who took up many concubines who bore many children to the emperor, the Hongzhi Emperor had only one Empress during his lifetime. Coupled with the fact that Empress Zhang had only two sons (one of whom died in infancy), the Hongzhi Emperor was left with only one nominee to succeed him. After the emperor died in 1505, he was succeeded by his son, the Zhengde Emperor. The Zhengde Emperor died childless in 1521 and the throne had to be passed to a cousin from Hubei named Zhu Houcong, effectively ending the Hongzhi Emperor's own line of succession.”— wikipedia