To be "bethrothed" meant a state of marriage without cohabitation¹. Only a formal divorce could dissolve a bethrothal. So, Joseph would have known about Mary's vow of perpetual virginity. There's no way that Mary would have kept that from him. Yet, he "married" her anyway! This obviously implies Joseph's own vow of perpetual virginity!
When Joseph learned that Mary was pregnant (St. Matthew 1:19-20), part of his dismay was based on her vow of perpetual virginity - "Mary! What do you mean you're pregnant? I thought you took a vow to be a virgin for life? I was planning to do this along with you!"
Getting back to Mary, when she "said to the angel: How shall this be done, because I know not man?" (St. Luke 1:34, DRB), Mary was stating not only her present condition of virginity, but her intention of her future state (the Greek present tense of "know" implies current and future).²
Truly indeed this couple was planning a remarkable marriage!