At the start of Act IV, Prospero announces that he is going to allow Miranda and Ferdinand to get married. He has watched the way that Ferdinand has gone about all the work that he has assigned and he is pleased with what he sees of the young man.
But Prospero does have one warning for Ferdinand. He tells Ferdinand that he and Miranda must not have sex until they are actually married. He warns Ferdinand that if the two of them do have sex before they are married their relationship will not go well. Here is the relevant quote:
If thou dost break her virgin knot before All sanctimonious ceremonies may With full and holy rite be ministered, No sweet aspersion shall the heavens let fall To make this contract grow, but barren hate, Sour-eyed disdain, and discord shall bestrew The union of your bed with weeds so loathly That you shall hate it both.