In the first act of the play, Theseus tells Hermia she must marry the man her father has chosen. If she does not, only two options remain:
"Either to die the death or to abjure
For ever the society of men."
The first needs no explanation. It's the execution that Hermia's father has requested. The second is a reference to becoming a nun, as Theseus quickly spells out. Theseus acknowledges that some people believe nuns lead blessed lives. But he offers his own views -- that women are happier when they live out their lives as wives and mothers. In particular, he uses imagery of emptiness and coldness to describe the life of a celibate nun:
"…in shady cloister mew'd,
To live a barren sister all your life,
Chanting faint hymns to the cold fruitless moon."
He compares the married woman to a blooming rose, while he says the life of a nun is one spent "withering on the virgin thorn."
So Theseus presents his options as if there is really only one tenable one. If Hermia doesn't obey her father and marry Demetrius, she is choosing either death or an empty, unhappy life. Either way, says Theseus, these options are inferior to marrying her father's choice.
If Theseus's options seem harsh to us, we should keep in mind that Elizabethans viewed things differently. Marriage was arranged by the parents of the betrothed; children were expected to obediently marry whomever their parents selected.
As William Gouge argued in a book published a few years after the end of Elizabeth's reign, doing so was a sacred and civic duty. Marrying against your parent's wishes -- as Hermia proposes to do -- was a sin. Instead of refusing to accept a parent's choice of spouse, children were supposed to ask God to intervene and change their own feelings about the matter. They were supposed to pray to God that their feelings about the potential spouse would become more favorable. If, after doing this, they were still averse to their parent's choice, it might be acceptable to respectfully request that their parents rethink their choice:
"…[T]hey may in a reverend manner entreat their parent to forbear to press that match, and to think of some other."
But even then, the child must feel bound by the parent's decision. If a child marries against her parent's wishes, Gouge says, she will be cursed by God.
Gouge, William. 1622. Of Domestical Duties.
Mortimer, Ian. 2014. The Time Traveler's Guide to Elizabethan England. Penguin.