A good girl saves herself for the man who is willing to commit to her. She doesn't sleep around while waiting for him to arrive. She doesn't have oral or anal sex while technically remaining a virgin based solely on the fact that she still has an intact hymen.
After she is married, a good girl has sex with her husband willingly and often. She tries new things in the bedroom (or the living room, the kitchen, the shower, the car, the backyard under the stars, etc.). She makes every reasonable effort to please her husband in bed, to fulfill his desires and keep him satisfied. In turn, if she has married a kind, generous man, he will reward her by making sure her sexual needs are met as well.
A married good girl won't withhold herself from her husband. She proves her chasteness by remaining faithful to her spouse, by absolutely never having an affair, no matter what or who the enticement may be. She doesn't prove it by refusing marital sex.
A woman who is cold to her husband in bed isn't a good girl; she's a bad wife.