I understand that Mary’s response of “knowing no man” is indicative of something .The pregnancy was in the future ,and what could be more normal in a marriage .I just am not sure it is “clear” that she made a vow of celibacy . She was betrothed, a bit like engaged today. However, back then it was more.They were legally married, papers signed ,dowry paid etc.etc… The only thing yet to be done was the wedding ceremony and consumation.This could be done at any time. I believe (not totally sure) they could have had sex ,and it would not have been fornication , for they were married (just no ceremony yet). So Mary was married , and was waiting for her ceremony, which Joseph could plan at any time. .She was in waiting and possibly asking herself , “when Joseph when ?”. A bit like today being engaged ,but with no wedding date, and time passes and passes.This could have been the reason for Mary’s response. She could not say "but I am not married yet ",because she was. She could have meant it was not consumated ,and "I have no idea when Joseph will come for me .It may have even been a plea to the angel ,a hinting , that “I am willing but you got to get Joseph in on the program , cause he is procrastinating and who knows when he will want to consumate”. A bit like today , Mary would tell the angel,"Sure ,I’ll have a child ,but I gotta get married first ( we have been chaste during our engagement). Again, Mary was married -betrothed ,so all she could say is I have not consumated ,or had the ceremony or have not known Joesph . I believe this is just as plausible a scenario as her vow of chastity . Both views have to be extrapolated from scripture .Thank-you for listening to my view .Yours is very plausible and coherent also. One thing we both can agree on , this is a perfect scripture to show her virginity , beyond a shadow of a doubt .If she had said , I am not consumated yet or had my “wedding ceremony” yet some could say maybe she had pre-betrothel sex etc. .This makes it very clear .She had sex with no man ,not even her husband.