but don't forget the massive and repeated emphasis that the early chapters of Proverbs place on avoiding the adulteress.
Correct, but the primary reason for these warnings is that a young man will not sin against another man. Same goes for Joseph with Potiphar's wife, who said he will not lay with her because Potiphar is a good master and "because you are his wife," (Gen. 39:9). Absent in both these examples is the notion that a young man has a "virginity" to be guarded.
Sin is always first of all against God.
Yes, but adultery is spoken of, even in the NT, as "defrauding" a brother.