There is a very interesting story which mentions as to how Shri Hanuman had a son. However, it was not out of marriage, but due to an incident which happened during his search of Mata Sita.
The mighty Hanuman crossed the ocean in search of Mata Sita and found her in Lanka. After meeting her, he destroys the Asokavan, deliberately gets captured and gives counsel to Ravan. But Ravan does not listen and sets Hanuman’s tail on fire. Hanuman grows in size, and sets Lanka on fire, thus destroying the confidence of the demons.
He then goes out into the sea, takes a dip in the ocean waters to put out the fire on his tail. At that time, a sweat drop falls from his brow, and is swallowed by a huge fish. The fish is actually a Gandharva women named Suvarchala, who was cursed to serve Mahiravan, the demon king and brother of Ravan, who resided in the Patal Lok. As she swallows the sweat drop of Hanuman, a son is born to her named Matysvallabh. However, Hanuman isn’t aware of this.
Later, after knowing the whereabouts of Mata Sita, Shri Ram and his Vaanar sena reach Lanka and the war starts. Ravan on the verge of losing the war calls for Mahiravan, his brother. He asks him to abduct Shri Ram and Lakshman and give them as a sacrificial offering to Goddess Kali in the Patal Lok.
Although Hanuman and Vibhishan tries their best to avert the abduction, Mahiravan kidnaps Shri Ram and Lakshman through sorcery. Hanuman sets out to the Patal Lok to save them both, and reaches the entrance of Patal Lok, wherein he sees a young boy who is half Vaanar and half Makar(fish) guarding the doorway. The boy stops Hanuman and a fight starts between the both. Both are equally strong and none is able to outwit the other.
At one time, Hanuman punches a mighty blow on the young boy, but he withstands it and stands back again. Surprised, Hanuman asks him the reason behind his strength and who is he. The boy replies, he is Matsyavallabh the son of Hanuman. Hanaman is taken back by these words as he is a Poorn Bramhachari and how could he have a son without his knowledge. He gets enraged that it is all a trick of the demons and runs forward to kill the young boy.
At that moment, Suvarchala comes to him and reveals the whole story behind the birth of Matsyavallabh. As he was the son of Hanuman, Matysavallabh had got power to grow mighty in just few days and get the strength of his father. Suvarchala also reveals that she was cursed by Matanga Rishi to be the slave of Mahiravan till Hanuman comes and frees her. Hanuman envisions all the events which happened through his Dhyaan and calms down and blesses them both. Suvarchala redeemed from the curse, goes back to Gandharva Lok.
Hanuman then goes and frees Shri Ram and Lakshman and the three of them fight Mahiravan and kill him. Later, Shri Ram comes to know of Matsyavallabh, blesses him, and
crowns him as the king of Patak Lok. Thus, this is the story of Shri Hanuman’s son
There are many temples dedicates to Shri Hanuman and Matysvallabh in the states of Madhya Pradesh, Andhra Pradesh, Odisha, Gujarat, Rajasthan and Maharashtra wherein they are worshipped as a Father Son duo.