Their love stories stand out because they were madly in love with each other. Aja and Indumati were parents of Dashrath and they were devoted to each other. When once they were spending some quality time, a garland of Narada fell on her and she died. It was predetermined and he fell into a pit of deep grief. He survived only for his son but after few years he passed away. There was no one else for him other than his dear wife Indumati. He like Shri Ram had never maintained an Antahpura because he couldn't imagine someone else in place of Indumati.
The love story of Mata Sita and Shri Ram needs no introduction. They were highly romantic and completely devoted from their souls to each other. Many times Shri Ram has said in the epic that he didn't want heaven without Mata Sita. Akampana had advised Ravana to abduct Mata so that Shri Ram could be defeated as he couldn't live without her. She didn't want to stay in luxuries of the palace of Ayodhya and happily came to jungle with him and she was the happiest there because her husband was with her till her abduction and both were miserable without seeing each other. Their devotion towards each other is remarkable and an inspiration.
Uttar Kand is considered an interpolation but even if it's considered as part of original Ramayana, it's evident that Shri Ram had forsaken the queen but not his wife. Like his grandfather Aja, he also never maintained an Antahpura. He had one and only one wife and she was Sita. He kept doing penances, did Yajnas with her statue with him and his mind was never at peace ever again. He missed her so much. Shri Ram had threatened mother Earth to return Mata to him and he pacified only when he was ensured he would reunite with her.
Kusha and Kumudvati are a little different story. Kusha was already married to many Princesses as per Ananda Ramayana and Raghuvansh.
In Ananda Ramayana, his first wife was Champika, daughter of Maharaja Bhurikirti of Eastern Lands. She had chosen serious and skilled in archery Kusha in Swayamvar and he was an extremely shy prince. He had nine daughters with Champika but no sons. Their daughter Hema was married to Prince Chitrangad of Avanti.
When Shri Ram went to his divine abode, as narrated to him by Rishi Agastya, Kusha got a second wife named Kumudvati and she gave birth to his Eight sons and the eldest one was Atithi who succeeded him on the throne of Ayodhya. Their daughter was called Champakmalini.
In Raghuvansh as well, Kusha is called married to many princesses and he was on a pleasure tour in Sarayu river with them when he lost his golden bracelet and it was found by Kumudvati. When he decided to attack Nagas of Sarayu river to get his bracelet back which was a gift from Shri Ram, Kumud offered the bracelet along with Kumudvati’s hand in marriage and he accepted her hand and suddenly the devas blessed the union by showering flowers.
Kumudvati was crazily in love with Kusha. She was highly devoted to him and they shared passionate love. He made their son Atithi as Prince Regent and wished to ascend the throne after he's gone when he left to fight demon Durjaya to help Indra. He was fatally wounded after slaying Durjaya and died. Kumudvati not being able to handle the pain follower her husband in death. Atithi succeeded him on the throne.
Kusha and Kumudvati are typical royalty of Ancient India. His love story had proper happiness and enjoyment which his father and great grandfather couldn't have. Kusha didn't suffer separation from his spouse and probably it was karmic justice.
Sources- Ananda Ramayana and Raghuvansh by Kalidas