Answer:
STDs are more readily passed male to female as opposed to female to male because the male is as a requisite to sex sharing his bodily fluid (semen) directly with her. That means that any fluids are likely to enter into an internal cavity of hers.
On the other hand, any bodily fluids she releases, which are not requisite to sex, are likely to stay on the outside of the man's body because they will just go onto the skin of his genital area.