Both.

Whitney stared as her boyfriend thrust himself deep into the redhead. Her muffled moans and cries were barely contained by his hand. Whitney stared as the girl changed, becoming sexier and sluttier. All the fat from her body moved to her breasts, ass, and thighs. Her toned stomach became more so. Her hair grew longer and redder, and her clothes became sluttier.

As Whitney watched, a new guy appeared behind her. The first she knew about it was when he stuck his fat cock between her thighs and began fucking her. She arched her back and moaned, savoring the feeling as he lifted her from the sand and bounced her on his cock. She began thrusting with him, and soon he came.

Whitney glanced over at where her boyfriend and the redhead were fucking, seeing her on her back with her boyfriend's penis imbedded in her. She turned to the stud and began fucking him in earnest, riding him and kissing him, their hands roaming across each other's bodies. It was the best sex she had ever had. She came again and again, only to keep fucking him harder. For hours they fucked, knowing nothing but sex.