SB 9.10.54
eka-patni-vrata-dharo
rajarsi-caritah sucih
sva-dharmam grha-medhiyam
siksayan svayam acarat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Ramacandra took a vow to accept only one wife and have no connection with any other women. He was a saintly king, and everything in His character was good, untinged by qualities like anger. He taught good behavior for everyone, especially for householders, in terms of varnasrama-dharma. Thus He taught the general public by His personal activities.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Eka-patni-vrata, accepting only one wife, was the glorious example set by Lord Ramacandra. One should not accept more than one wife. In those days, of course, people did marry more than one wife. Even Lord Ramacandra’s father accepted more wives than one. But Lord Ramacandra, as an ideal king, accepted only one wife, mother Sita. When mother Sita was kidnapped by Ravana and the Raksasas, Lord Ramacandra, as the Supreme Personality of Godhead, could have married hundreds and thousands of Sitas, but to teach us how faithful He was to His wife, He fought with Ravana and finally killed him. The Lord punished Ravana and rescued His wife to instruct men to have only one wife. Lord Ramacandra accepted only one wife and manifested sublime character, thus setting an example for householders. A householder should live according to the ideal of Lord Ramacandra, who showed how to be a perfect person. Being a householder or living with a wife and children is never condemned, provided one lives according to the regulative principles of varnasrama-dharma. Those who live in accordance with these principles, whether as householders, brahmacaris or vanaprasthas, are all equally important.