Matthew 1:19 (Putting away secretly)

19. not willing to make her a public example ― If a betrothed woman was found to be pregnant by someone other than her betrothed, it was considered adultery, which carried severe legal and social consequences, including public shaming or stoning (Deuteronomy 22:23-24). Since Joseph had only performed erusin with a na’arah, it would seem impossible for Mary to have conceived. To accuse her of adultery would involve a public accusation and trial which would expose Mary to community scrutiny and punishment.

put her away secretly ― Secret only in the sense of the public’s eye. The term ‘put away’ means to end the arrangement (typically through a divorce certificate), which must be done in the presence of two to three witnesses. This minimizes public knowledge. By opting for a quiet divorce, Joseph shows a desire to avoid a public scandal. If the child was suspected to not be his, then he is properly choosing a balance of justice and mercy – upholding legal propriety while mitigating harm to Mary.

❑ Judah suspected Tamar (daughter of a priest) of adultery and wanted to make her a public example. He later found out it was his own child. Joseph brings reconciliation to this. Joseph (from the tribe of Judah) suspects Mary (daughter of the Levitical priestly line) of adultery and does not want to make her a public example. He later found out it was his own child.

❑ Anyone who makes the accusation that their betrothed was not a halachic virgin would be subject to lashes and a fine – if he was wrong[1].

❑ Under mi’un, (refusal) one who is engaged under the age of 12 can walk away from the marriage with no questions asked, and there is no divorce certificate required[2]. The marriage is considered invalid from the start if the girl performs mi’un. She is free to marry anyone else without restrictions.
[1] Mishnah Ketubot 1:6, Babylonian Talmud Ketubot 46a-46b, Even HaEzer 68:1-3 [2] Yevamot 13:1, Yevamot 107a-109b, Even HaEzer 155.