Secondly, the text says nothing about Mary making some vow, either implicitly or explicitly. That is something which Catholics have read into the text.
Thirdly, the understanding of "betrothed" found in the quoted excerpt is quite different than how the term has normally been used. The Reformation Study Bible has this footnote on Matthew 1:19: "Joseph . . . resolved to divorce her quietly. Engagement was almost as binding as marriage, and infidelity during betrothal made divorce almost obligatory."