There are actually many versions about the way Dionysus was born but you will allow me to only convey the juicy one. Just like every juicy myth involving Zeus, this too includes his adultery and flirting with mortals as always.
This story begins in Thebes with the daughter of Kadmius and Harmony, Semelis. Her beauty attracted the erotic gaze of the Father of the Gods and she gave herself to him in Kadmius’ palace. However, Hera who was informed of her husband’s adventure swore to annihilate her love rival (like that was a first). Thus, she took the form of a house-helper and convinced the easily persuaded Semelis to ask of her God lover to show her all his power.
Without any suspicion, Zeus’ lover first made him swear that he would succumb to her one and only request and then revealed what it was. Zeus willingly promises to fulfill any wish but once he hears what she wanted, he tries to convince her to let go of this demand. She insists, though, and Zeus has no other choice than to stand by his word.
He appears before her in all his power, on his chariot with all his thunder around him. One of his bolts unintentionally hits her and she dies immediately, while also setting the palace on fire.
Semelis was already pregnant to Dionysus but because her pregnancy was not as long as it should have been, Zeus cut her and from her insides extracted the little God, whom he kept sewed in his thigh to grow.
Source : Ρίσπεν Ζ., Ελληνική Μυθολογία, Εκδόσεις Τριήρης, Τόμος 3ος, σ.396