Your boyfriend hadn't always been this built. When you first got together he was a tall skinny twink, but you saw potential. You made sure to always get him in the gym together, forcing him to build muscle and size. The best part was the warmup.
You made sure before each workout you had him bent over in bed, ass out so you could check on his progress and stretch him out. You got to enjoy his growing body, a sexy stud, fucking his ass and giving him a boost of testosterone he needed. It got your heart rates pumping, ready to put your all into your exercise while he got to enjoy the feeling of your cum leaking from his crack throughout your workout.
He began to get more attention from his body, his confidence growing with his muscles. You hadn't realized the effect it had on him, his personality becoming more cocky, beginning to get more ideas in his head that he could end up with better. He ended up leaving, thinking he didn't need you anymore and that he could be on his own. It hurt, but in your head you knew it was probably for the best, thinking maybe he'd find someone better to put up with his attitude.
That's when he came crawling back, begging you to take him back. He'd realized the effort you put into him, how much you put up with him, realizing no one else would do the same. And worst of all, he needed someone to fuck him before his workouts. He was on his knees, constantly texting you, begging you to fuck him before he went to the gym.
He needed to earn his way back into your life if that's what he wanted. From now on he wasn't anyone to you, just your toy to grow, a toy to fuck before each workout. He obeyed without question, obediently in your bed each day before the gym, letting you use him as you will.