Genital tag is just like phone tag only it involves sex. It occurs when a man and a woman try to have sex but each person's genitals are in a state of dysfunction at exactly the same time that their partner is ready. (i.e. She can't get her juices flowing when he has his erection and and he loses his erection as soon as she gets her juices flowing.)
Brian Cass and his wife gross each opther out a lot; they played genital tag for the duration of their honeymoon.