UTTARA KANDA is a PRAKSHIPTA - an insertion made at a later date. As the episode of banishment of Sita finds place in this Kanda, it has to be rejected.

In view of the following contradictions in the UTTARA KANDA, I think the entire Uttara Kanda is a PRAKSHIPTA, an insertion made at a latter date.

1.   The story of Srimad Ramayana ends with Sri Rama's coronation as the King of Kosala Kingdom.  Sage Valmiki describes in the concluding chapter of Yuddha Kanda that having enjoyed the kingship for ten thousand years, Sri Rama performed a hundred horse-sacrifices.   It was described very briefly about the happy life of the people of Kosala kingdom during the reign of Sri Rama.
   

सर्वे लक्षणसम्पन्नाः सर्वे धर्मपरायणाः || दशवर्षसहस्राणि रामो राज्यमकारयत् | (Yuddha Kanda 128 Sarga 106 Sloka)

"All the people were endowed with excellent characteristics. All were engaged in virtue. Rama was engaged in the kingship thus for Ten thousand years."

2. This was followed by PHALA SRUTI. In the fag end slokas of Yuddha Kanda PHALA SRUTI, the result of reading Srimad Ramayana, was described.

धर्मयं यशस्यमायुष्यं राज्ञां च विजाअवहम् || आदिकाव्यमिदं चार्षं पुरा वाल्मीकिना कृतम् | पठेद्यः शृणुयाल्लोके नरः पापात्प्रमुच्यते || (Yuddha Kanda 128 Sarga 107-108 Slokas)

"In this world, whoever person reads and listens to this foremost lyric derived from the speech of a sage, which is endowed with righteousness, conferring fame and longevity, fetching victory to kings and as written at first by Valmiki, that person is delivered from all misfortune."

श्रुत्वा रामायणमिदं दीर्घमायिश्च विन्दति | रामस्य विजयं चैव सर्वमक्लिष्ठकर्मणः || (Yuddha Kanda 128 Sarga 112 Sloka)

"On hearing this epic of Ramayana and all the episode of victory of Rama, who was unweary in his actions, a person gets longevity to life."

विनायकाश्च शाम्यन्ति गृहे तिष्ठन्ति यस्य वै | विजयेत महीं राजा प्रवासि स्वस्तिमान् भवेत् || (Yuddha Kanda 128 Sarga 116 Sloka)

"Whoever carefully listens to the epic in his house, all obstacles come to an end. A king conquers the earth. A person staying away from home, fares well."

In all Hindu Paraayana texts it is a tradition to incorporate the PHALA SRUTI, the result of reading a Sacred Text, in the end of any PAARAYANA but not in the middle.

Hence, if the PHALA SRUTI was added at the end of Yuddha Kanda of Srimad Ramayana, it indicates that Sage Valmiki in fact closed his writing on Srimad Ramayana with that Sarga. Consequently, UTTARA KANDA can be concluded to be a PRAKSHIPA, and insertion made at a latter date,

3. While trying to stop Ravana in ordering killing of Sri Hanuma,  Vibhishana says there was no precedence,  of killing messenger (Sundara Kanda).
   

वैरूप्याम् अन्गेषु कश अभिघातो | मौण्ड्यम् तथा लक्ष्मण सम्निपातः | एतान् हि दूते प्रवदन्ति दण्डान् | वधः तु दूतस्य न नः श्रुतो अपि || (Sundara Kanda 52 Sarga 15 Sloka)

"Some of the punishments to an envoy are-deforming the limbs, striking with a whip, shaving the head and impressing marks on the body. Indeed, we have not heard at any time of killing a messenger."

Vibhishana was saying just One month before Great Battle that took place in Lanka. He was saying that till then there was no precedence of Killing a messenger.

However, it was narrated in the 13 th Sarga of Uttara Kanda about killing of the messenger of Kubera by Ravana. This incident stated to had been took place at the time of Ravana's commencement of wars on Devatas, Yakshas, Gandharvas, etc, at his younger age.

Had Ravana really killed a messenger of Kubera, Vibhishana might not had said that there was no precedence of Killing a messenger.

Hence, the Uttara Kanda is PRAKSHIPTA

4.     In the fag end slokas of Yuddha Kanda it was described that While Rama was ruling the kingdom, people survived for thousands of years, with thousands of their progeny, all free of illness and grief. And,  old people did not perform obsequies concerning youngsters.
   

निर्दस्युरभवल्लोको नानर्थः कन् चिदस्पृशत् | न च स्म वृद्धा बालानां प्रेतकार्याणि कुर्वते || (Yuddha Kanda 128 Sarga 100 Sloka)

"The world was bereft of thieves and robberies. No one felt worthless nor did old people perform obsequies concerning youngsters."

It is said by 'Valmiki' that in Sri Rama's reign there were no premature deaths in his kingdom. It would be unbearable to a father, if his son dies before him. Any father wishes to die in the hands of his son. It was stated in above sloka that while Sri Rama was ruling the kingdom of Kosala, no youngster died before his father died consequently old people did not perform obsequies concerning youngsters. .

However, a premature death of a son of a Brahmin was described in the 73 - 76 Sargas of Uttara Kanda.

It happened that a certain Brahman's son died in a premature death. The bereaved father carried his body to the gate of the king's palace, and placing it there, cried aloud and bitterly reproached Sri Rama for the death of his son, saying that it must be the consequence of some sin committed within his realm, and that the king himself was guilty if he did not punish it; and finally threatened to end his life there by sitting on a dharana (hunger-strike) against Sri Rama unless his son was restored to life.

Sri Rama thereupon consulted his council of eight learned Rishis, and Narada amongst them told him that some Shudra among his subjects must have been performing Tapasya (ascetic exercises), and thereby going against Dharma (sacred law), for according to it, the practice of Tapasya was proper to the twice-born alone, while the duty of the Shudras consisted only in the service of the "twice-born". Sri Rama was thus convinced that it was the sin committed by a Shudra in transgressing Dharma in that manner, which was responsible for the death of the Brahmin boy.

So, Sri Rama mounted his aerial car and searched the countryside for the culprit. At last, in a wild region far away to the south he espied a man practicing rigorous austerity of a certain kind. He approached the man, and with no more ado than to enquire of him. That person inform himself that he was a Shudra, by name Sambuka, who was practicing Tapasya with a view to going to heaven in his own earthly person. Sri Rama without so much as a warning, expostulation or the like addressed to him, cut off his head.

At that very moment the dead Brahman boy in distant Ayodhya began to breathe again. Here in the wilds the Gods rained flowers on the king from their joy at his having prevented a Shudra from gaining admission to their celestial abode through the power of the Tapasya which he had no right to perform. They also appeared before Sri Rama and congratulated him on his deed. In answer to his prayer to them to revive the dead Brahman boy lying at the palace gate in Ayodhya, they informed him that he had already come to life.

This episode of Sambuka described in 73 - 76 Sargas of Uttara Kanda contradicts the statement of Sage Valmiki that in Sri Rama's reign there were no premature deaths.

Thus, UTTARA KANDA can be concluded to be a PRAKSHIPA, and insertion made at a latter date,

5.   Srimad Ramayana was written much earlier to Mahabharata.  In the 272-289 Sections of Vana Parva of Mahabharata, the story of Sri Rama was narrated to Yuddhistara by Sage Markandeya.   Though the story contains minor variations compared to the story told in the Srimad Ramayana, those episodes describe the story of Sri Rama in full.  
   

However, the sage Markandeya ends the story of Sri Rama in 289 Section of Vana Parva of Mahabharata with the coronation of Sri Rama as the king of Kosala Kingdom. No mention was made therein the story of UTTARA KANDA.