In the story, ‘The Birth of Kartikeya’, the dual nature of Puranic symbolism can be seen very clearly. Kartikeya is said son of Agni and Swaha in many unnatural ways. Kartikeya is said to have five faces or sometimes six faces. From generation to generation, storytellers were telling and changing stories according to current social conditions. Puranas are known as historic record of past.
As stated in all puranas, writer of Mahabharata, sage Vyas collected historical accounts and taught his six pupils. Hindus consider Puranas more sacred than other scriptures because Vyas gave more emphasis on Puranas.
The legend of the birth of Kartikeya is told in many Puranas and in Mahabharata too. In the first chapter of Mahabharata, which is known Adi Parv, Kartikeya is described as Anal's son. Anal belonged to Agni clan. In another chapter in Mahabharata, Kartikeya became leader of the Devas who wanted to kill demon Tarkasura. Because of Brahma's blessings to Tarakasura, only Shiv and Parvati's son could kill him. So Devas were waiting for their son to be born.
According to one story told in Matsya Purana - when Shiva was copulating with Parvati, Devas interfered their pleasure with Agni as their leader. Due to this the copulation did not complete. Enraged Parvati cursed Agni to bear the semen of Shiva which was dropped elsewhere. Bering the semen Agni became very hot and wandering from place to place to cool down. Nobody was willing to bear the semen. Eventually Kratika offered to bear it, therefore the boy born out of it was known as Kartikeya.
Later Matsya Purana also tells that Kartikeya was playing in Sarapatvan near Ganges. Indra, the leader of Deva, had become scared of Kartikeya's strength, so he sent Matrka to kill Kartikeya, but Kratika saved the baby. When Parvati saw the beautiful baby with Kratika, Parvati took the baby from her and adopted him.
In another story told in Skand Purana and Mahabharata – Agni enamored on wives of seven sages. He wanted to have sex with them. These women were very poise and Agni could not have them; therefore, Agni was wandering in forests heated with lust loosing his power. Daksh's daughter, Swaha came to know about Agni's lust on wives of sages.
Since she loved Agni, she decided to sooth Agni, and she started to take form of every woman one by one. This way she copulated with Agni but she could not take form of the Arundhati the seventh wife. She collected Agni's semen on a lotus leaf and floated it in river Ganges. Kratika came there to bathe and happened to bear the semen. Later Kartikeya was born with six faces.According to some other stories Kratika offered herself to bear the semen when everybody refused. There are many variations of the story but these three are the main.