Mata Sita was Ayonija (born without womb) and she was found by Rajrishi Sheerdhwaj Janaka when he was doing ritual ploughing in Mithila. He adopted her and gave her to his Chief Queen. Later, she was wedded to Prince Ram of Ayodhya when he proved himself worthy of her by picking up Lord Shiva’s bow.
Mata Sita was happy in her marriage, she was very happy because she fell in love with her husband after marriage. Their marital life was very blissful because their minds, habits, nature matched with each other and they both were very respectful of each other.
Even when Shri Ram was exiled, she was unhappy that he was not taking her with him in exile. She argued and then started crying, then only he agreed to take her in exile. In Jungle, she was happy, very happy because she had the full focus of Shri Ram on her. They were living like two free birds in Jungle. Just the company of her husband was enough to make her happy.
The problems in her life started when Ravana abducted her and when she was rescued, she gave Agnipariksha because Shri Ram said some words which hurt her though he later explained he had full faith in her because she was saved by her moral power but it was needed because the people would never stop judging her.
Uttar Kand is considered an interpolation in original Valmiki Ramayana and in this source, he sent Maithili in the hermitage of Maharishi Valmiki when she was pregnant as his people wanted him to leave his queen and marry again. But Shri Ram although sent his queen away but he didn't remarry because Queen belonged to their people but she was wife of Shri Ram, it was his personal relationship and he proved his people, he sent his queen away not his wife. They didn't want the queen but he wants his wife and no one could take her place , not even for strategic alliances which Kings often did. Shri Ram is the only King who never had any harem or looked at any women and other than Mata Sita. He was absolutely devoted to her but Mata Sita refused to come back because she preferred her self respect over becoming queen again.