Mary's perpetual virginity is not only not elaborated on in Scripture - it isn't there.
The trinity is.
I agree that perpetual virginity is not in the scripture wih nearly the same lucidity as the Trinity; the comparison was with the lack of elaboration on Mary's seemingly illogical response in Luke 1:34, and similar lack of elaboration on some finer trinitarian aspects, which caused the great trinitarian heresies of the Early Church and even today.