It's not?? I think it's evident that we become in essence one flesh during sex. Although that's probably not the entire meaning of the verse.
I would think that given the evidence in verse 25 that they had no relations, interpreting Joseph "taking" Mary as his wife to mean they had relations is contradictory.
In fact, one could argue that the fact that Joseph married her without the usual wedding night festivities establishes that this is a diffeent kind of marriage.
SD