The rejection of Hero happens as she and Claudio have come before the Friar (plus all the characters of the play and the audience) to be married in Act IV, scene i. The actual line of accusation is this:
Not to be married, not to knit my soul
To an approved wanton.
To which Leonato at first tries to appease Claudio by suggesting that if he, Claudio, has been the one to "de-flower" Hero, then -- But he is cut off by Claudio, who makes it clear that it is not he that has taken Hero's virginity and that she is "more intemperate" than "those pamper'd animals/That rage in savage sensuality."
Leonato then attempts to have Don Pedro speak against this, but, of course, Don Pedro stands behind Claudio. Leonato says, "Are these things spoken, or do I but dream?" And he "charges" Hero to answer truly who the man was that she "talk'd" with "out at [her] window" the night before the wedding. Hero, of course, denies -- truthfully -- talking with any man. And, as Claudio and Don Pedro storm out of the wedding, Leonato says, "Hath no man's dagger here a point for me?"
Once Leonato is left with his daughter, niece, the Friar and Benedick, he sees that Hero has fallen into a deathly faint and welcomes it. He says:
Death is the fairest cover for her shame
That may be wish'd for.
Leonato confirms, with these words, that he believes the Prince and Claudio over his daughter and that she is better off dead. He then has a very long speech about Hero's shame (and his own, since she is his issue), all based upon the assumption that, since honorable men have accused her, she must be guilty.
Finally, the Friar and Benedick (also men) are able to convince Leonato to give Hero's story a chance, and he swears that, if she has been accused falsely:
The proudest of them shall well hear of it.
Time has not yet so dried this blood of mine,
. . .But they shall find, awak'd in such a kind,
Both strength of limb and policy of mind
. . .To quit me of them throughly.
And from this point, Leonato is Hero's strongest defender.