The first scene of Act IV would have been a wedding scene if not for the villainy of Don John. However, when Leonato brings Hero up before Claudio so that the Friar (like a Catholic priest) can perform the wedding, Claudio refuses her:
There Leonato, take her back again.
Give not this rotten orange to your friend.
She's but the sign and semblance of her honor. (29-31)
During the Renaissance, most men wanted to marry women who were virgins. A woman who had slept with another man was like rotten fruit to them, worthless and unappealing. Even one act of sexual intercourse would be enough for the woman to lose all of her honor and be labeled as a whore or prostitute. When Hero hears the false accusation, she blushes. Her face turns red because she is upset and confused. But Claudio interprets her blushing as a sign of her guilt and continues to accuse her of not being a maid, of not being a virgin.
Hero finally gets enough breath to ask Claudio about why he believes this about her. Claudio responds with mythological references. Although Hero had appeared to him to be as chaste as Diana, the virgin goddess of the moon, Hero was actually as sexually loose as Venus, the goddess of love who is well known for her sexual affairs (lines 55-59).
Leonato is, at first, in shock at hearing this accusation as well. He finally asks the Prince, Don Pedro, to respond to this wild accusation. But Don Pedro only confirms Claudio's words. In fact, Don Pedro even refers to Hero as a prostitute or whore ("common stale" in line 63); and he then explains to Leonato how, during the night, he and Claudio saw Hero with Borachio (referred to as "a ruffian" in line 90).
To hear the respected Prince say these words and confirm the accusation shocks both Leonato and Hero even further. The Prince, after all, is supposedly the most honorable man in the region. He would not lie. A gentleman or minor aristocrat would not even dream of contradicting a prince.
After Claudio says more hurtful words to Hero, she faints. The shock has been too much for her. Don Pedro and Claudio then exit, leaving all of those who had come to see the wedding shocked and confused.