Why Did St. Joseph Want a Divorce?
Why did Joseph want to divorce Mary?
Mary and Joseph were betrothed, but they had not yet begun to live together. As the New American Bible's commentary on Matthew 1:18 states:
Betrothal was the first part of the marriage, constituting a man and woman as husband and wife. Subsequent infidelity was considered adultery. The betrothal was followed some months later by the husband’s taking his wife into his home, at which time normal married life began.
Since it would have been several months before Mary and Joseph lived together and lived a normal marital life, it would have been obvious to everyone that the child Mary carried was not Joseph's. Joseph obviously knew the child was not his, so he would have been very hurt to discover that Mary was pregnant.
Joseph was a human being with emotions and feelings; finding your betrothed is pregnant and the child is not yours is quite an emotional hit. Like most engaged people who believe their partner has cheated on them, Joseph wanted to end the relationship. According to the law, he could have had Mary stoned for adultery (Deut. 22:21-23), but he loved Mary and didn't wish harm to come to her (Matt. 1:19).