At the start of creation Brahma divides into a man and a woman, and the woman is Saraswati. He is so taken with her that faces form on every side of his head to follow her, and from their union comes the first Manu, Svayambhuva. The Purana knows how this sounds and answers the objection directly: what is right for humans cannot be applied to the gods when the world itself is being made. In the same passage the names Saraswati, Savitri, Gayatri, Brahmani and Shatarupa are said to belong to one goddess.
The Bhagavata keeps Saraswati as both daughter and wife but removes the stain. When desire for his own creation rises in Brahma he feels shame and gives up the body in which it arose; a new Brahma, in a new body, is the one who marries the goddess his predecessor made.
What is different here:
Adds shame and the abandoned body; the husband is technically a different Brahma from the father.
In the Shiva Purana Saraswati does not accept the creator's approach. She rejects him, curses him and leaves. The Shaiva text thus makes her the first of the many who humble Brahma.
Three Puranas make Saraswati simply Brahma's daughter, a maiden goddess who takes no husband. In these texts Brahma's wives, where he has them, are Savitri and Gayatri, kept distinct from her, and the Brahma Purana lists Gayatri and Savitri among his five daughters.
What is different here:
No marriage at all; Savitri and Gayatri are separate figures.
The Brahma Vaivarta and the Devi Bhagavata make Saraswati Brahma's wife and nothing else, brought forth from his spiritual essence rather than from his body, so there is no daughter and no incest to explain away.
What is different here:
Consort without the daughter relationship; the origin is from his essence, not his split body.
Vedic antecedent: Prajapati and Ushas (Aitareya Brahmana)
Pan-IndiaAll traditions
The oldest form of the motif has Prajapati take the shape of a stag to approach his daughter Ushas, the dawn, who has become a doe. The gods, appalled, gather their most fearsome parts to make Rudra, who pierces Prajapati; the creator is later restored. Wikipedia suggests the Puranic Brahma-Saraswati story descends from this, and a review of Doniger notes her tracing of how such creation-by-incest myths shifted in ethical emphasis from the Rigveda to the Jaiminiya Brahmana.
What is different here:
Prajapati and Ushas rather than Brahma and Saraswati; Rudra is created to punish the act, which the Skanda Purana later reuses as the reason for the fifth head.