Theseus first sides with Egeus and tells Hermia to obey him, and then changes his mind and overrules Egeus to tell her she can marry Lysander.
Theseus either contradicts himself or changes his mind when he decides to overrule Egeus in Act 4 after siding with him in Act 1. He flatly says that he doesn’t really believe the lovers’ story, but he still decides to allow them to marry. Also, Theseus’s relationship with Hippolyta contradicts the role of women which he seems to espouse for Hermia.
When Egeus first comes before Theseus to request intervention in his daughter’s affairs, Theseus plainly takes his side. He basically tells Hermia that as her father, Egeus can tell her what to do and she has no choice but to obey him.
What say you, Hermia? be advised fair maid:
To you your father should be as a god;
One that composed your beauties, yea, and one
To whom you are but as a form in wax
By him imprinted and within his power
To leave the figure or disfigure it. (Act 1, Scene 1)
He tells her that she can either marry Demetrius, the man of her father’s choosing, or become a nun. If she does not obey her father’s wishes she can be put to death. A woman has no rights. She is her father’s do with as he will.
A lot of really interesting things happen to the lovers during their time in the woods. Most of the silliness that ensues is as a result of the intervention of fairies, specifically through Puck. The lovers get paired off and unpaired at his will in order to wreak havoc. In the end, however, they end up re-paired exactly as they were when they went into the woods: Hermia and Lysander, and Helena and Demetrius. Demetrius, despite the ruling of Theseus and the will of Egeus, does favor Helena.
When Theseus hears of the lovers’ exploits, he is not exactly convinced.
More strange than true: I never may believe
These antique fables, nor these fairy toys.
Lovers and madmen have such seething brains,
Such shaping fantasies, that apprehend
More than cool reason ever comprehends. (Act 5, Scene 1)
You would not expect him to change his ruling, after he so strongly worded it the first time. However, he seems to be influenced by the fact that today is his wedding day. Love is in the air, so to speak. He pronounces the match, and basically tells Egeus that he needs to go along with it.
Why does Theseus so easily change his mind? We see that his relationship with Hippolyta is not at all like what he described for Hermia. Hippolyta is far from the obedient, simple woman with no free will. He wooed her. She is free to speak her mind. He treats her more like an equal. Perhaps on his wedding day Theseus has come to understand the value of a woman being able to make her own choice.