‘Now the birth of Jesus Christ came about in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found with child of the Holy Spirit, and Joseph her husband, being a righteous man, and not willing to make her a public example, was minded to put her away privately.'
The verse opens with what almost seems to be a public announcement. This is what we would expect for the birth is of Jesus the Messiah, and how it came about is thus to be seen as important. Note that Mary is not seen as doing anything positive towards the child's conception. It is simply seen as something that happens to her. She was ‘found with child'. All is of God's activity through the Holy Spirit, and she remains secondary. After that Joseph takes over. Unlike the ancient myths where gods mated with earthly women there is no suggestion here of any kind of sexual activity, even spiritual sexual activity. Indeed in Jesus' eyes (and Matthew's eyes) heavenly beings do not engage in such activity, for that is very much an earthly phenomenon (Matthew 22:30), while what happens here is heavenly.
This lack of sexual activity is confirmed by the phrase ‘ek Pneumatos Hagiou' which, apart from its being without the article, parallels the description of the four women in the genealogy (ek tes Thamar; etc). The Holy Spirit is thus seen as cooperating with Mary in the conception and birth, not as impregnating her.
Note Matthew's great emphasis on Joseph's side of things, and this to such an extent that he puts Mary deliberately into the background, and plays down her part in things. This being his aim it is not surprising that he tells us nothing about the Annunciation and other activities in which Mary was involved. It would have placed too much attention on her and diverted his readers' thoughts away from his main purpose, which was that of establishing Jesus as the heir of Joseph, and thus the titular son of David, even though at the same time he was emphasising His birth through a virgin.
Mary was at the time betrothed to Joseph, who was the heir to the throne of David, and thus a man of high honour from a proud family. Betrothal was a binding state from which it was only possible to be released by divorce or death. It was at betrothal that the marriage covenant was signed and sealed, and all settlements agreed on. The wedding was only the final confirmation. But it would not have been seen as acceptable in the best families that sexual intercourse take place during this period. She would still be living at her father's house, awaiting the marriage. Indeed Joseph and Mary may well have had little to do with each other. Their marriage would have been arranged.
It is apparent that she had given him no notification of the pregnancy, but eventually the fact would have to come out, and the expression ‘she was found with child' may possibly express this idea. Once this was clear her parents no doubt contacted Joseph and informed him of the situation. Recognising the situation as he saw it, and being a ‘righteous man', that is, one who would do the right thing, he then determined to divorce her. It was not a matter of having an option. For him not to do so would bring disgrace on his name and on his family, and would be to be in breach of the Law and of public decency.
It would have been a very ‘liberal' minded man who would not have done so, and it would have revealed one who would not have been respected in the best circles, for it would have been to go against the very principles of the Law which was that she now ‘belonged' to the man who had ‘known her'. She had been made one with him. (See 1 Corinthians 6:15-16. This is also confirmed in the Mishnah). Love would thus not have come into it for a man in Joseph's position. It would have been even more so if she had been raped.
But being also genuinely righteous in a godly fashion, in a way exceeding the righteousness of the Scribes and Pharisees (Matthew 5:20), he did not wish to bring her into total and open disrepute by a public investigation (compare Numbers 5:11-31 for such an investigation, although that was where a child was not involved), so he decided to come to an arrangement for the divorce to proceed privately. This would involve the granting of a certificate of divorce before two witnesses and her then remaining at home in her father's house until a suitable marriage could be arranged with someone else. He would probably by this forego his right to recover marriage settlements and confiscate her dowry, but he was a compassionate man and did not consider such things. In view of the fact that he knew that the child was not his, which emphasises the fact that he had not had sexual relations with her, no trial was necessary unless he wanted one. The matter could thus be quietly resolved, with as little public shame as possible to Mary. She would then be able to accept any offer that she might receive, probably from an older close relative looking for a nubile second wife who would recognise her place. That would be the best that she could hope for.