(The question here made a bunch of assumptions, but ... let's give this a go ...)
The Gemara says that when men and women were in the Temple together, there was kalut rosh -- best translated as "disinhibited behavior" -- and thus they created a separate section for them.
Rabbi Moshe Feinstein's opinion was that a mechitzah serves, essentially, two functions:
A.) To keep the genders from mingling; and a wall sets a stronger tone about this than just asking people to sit separately.
(If this was the only reason, then a clear glass wall would be okay.)
B.) The Talmud says a man shouldn't pray near an inappropriately-dressed woman. Many a synagogue has female visitors who are baring "sleeves and more."
Rabbi Feinstein thus advised a mechitza that was opaque up to five feet -- at which point the only thing visible from the men's side would be the women's heads, assuming an average women's height; and women's heads being visible is not a problem.
Now what about the women praying in the presence of inappropriately-dressed men? I've been to a lot of synagogues, and the dudes are usually covering their skin okay.
Thus, we have a symmetric problem solved by any wall, and an assymetric problem solved by one-way glass.
The other critical advantage of one-way glass: if a man can't see the women's side, what's he missing vis-a-vis the worship? If the woman can't see the men's side, she can't see the main parts of the service.