Verse 20 "such is the way of an adulterous woman" is certainly not something the writer would consider to be "too wonderful" for him. Verses 19 and 20 are parenthetical verses while verse 20 goes back to verse 17 ---
I still don't see how 20 relates to 17. But, be it as it may, "the way a man with a maid /almah/" hardly suggests virginity or chastity for that word. It also does nothing to explain Barnes' false claim.