Is the penis rendered impure after penetration into the vagina due to vaginal secretions that are discharged during a woman's arousal along with pre-seminal fluid (madhy), and what are the implications of this for rendering impure anything the man or his wife touches before ghusl?

The man's ذكر (male organ) is not considered impure after intercourse, even if it comes into contact with the wife's secretions, because these secretions are considered pure. The default state is purity until proven otherwise. Moreover, semen (المني) is pure and does not necessitate washing.