The answer lies in the perception. If a woman adjust her bra it is because it has become uncomfortable. it may be pinching or rubbing on a tender spot.
A man adjusting his cup could be for the same reason, however it is the manner in which it is done. if it is done to provoke a response it is consider gross, if it is done to emphasize a statement it is gross.
However if a man adjust his cup in a discreet manner it is considered in the same manner as a women adjusting her bra.
By the way, a women has much better ways to attract attention than adjusting her bra, therefore it is never consider in the same light as adjusting a cup.