You're treating two different questions as though they were the same. We all agree that Scripture teaches the virginal conception. The question is whether the interpretation that Mary and Joseph later had marital relations adequately accounts for how Luke presents that virginal conception. In other words, my challenge is not:
"Does Luke teach the virgin birth?"
Everyone here agrees that it does. My challenge is:
If Mary expected future marital relations with Joseph and therefore future conception through Joseph, why does She explain Her difficulty by saying, "I know not man"? Why does Gabriel answer by explaining divine conception, appeal to Elizabeth's otherwise impossible conception, and conclude, "For with God nothing shall be impossible"?
Simply saying:
"Luke teaches the virgin birth"
does not answer that question. It restates the very passage under discussion.
The issue is whether the interpretation asserting future marital relations between Mary and Joseph adequately explains Luke's narrative. If it does, then please explain how Mary's appeal to "I know not man," Gabriel's answer, Elizabeth's miraculous conception, and "nothing shall be impossible with God" fit together with the claim that Mary expected future conception through Joseph. If it does not, then that is evidence that the interpretation may need to be reconsidered, even if that alone does not conclusively prove perpetual virginity.
