Sam had accepted your challenge because of his physical advantages. He was bigger, faster, and stronger than you were. But the former, as it turned out, was actually to your advantage.
Because Jousting was seen as weird, the two of you agreed to do it in secret. The Saturday after your school graduation, the two of you headed into your old school through an open window and made your way though the empty halls to the gym. You tied each others hands behind your backs with yarn. Tight enough to stay, but weak enough to loosen if you made an effort to escape. That way when the contest was over you could free yourselves without having to try to do it standing back to back. Rubber mats formed a makeshift arena, and with your arms tied the two of you bowed to each other.
"May the best man win...Tyla"
"You too Samantha"
After the two of you exchanged confident greetings, Sam came at you quickly. "Shit", you thought to yourself, "he's too fast!" But just as he was about to reach you, Sam slowed down dramatically. He crouched before thrusting at you, which gave you ample time to dodge. "Wait a sec, he's 6 inches taller than me! That means he can't penetrate me without crouching!" The look of consternation on Sam's face told you he'd just realized the same thing.
Of course, just because he couldn't get at your vagina from a standing position didn't mean his physical strength wasn't an advantage. He could always bump into you and knock you onto your back, or try and trip you up. Of course, you knew this too, so when Sam made an attempt to bump you you were able to dodge. He'd leaned too far forward and tumbled to the ground, landing on his ample breasts. He rolled over onto his back to try and get back up, but was struggling to do so without the use of his arms.
Sensing an opportunity you step in between his legs and flop down on top of him, managing to both his surprise and yours land your penis in his vagina on the first try. You'd won!
The two of you quickly freed your hands and silently began fucking. Perhaps all those cultures that still did this regularly had a point, because you didn't just feel the pleasure of having sex as you pumped Sam full of your semen, you also could feel an extra thrill. One of victory...of conquest. You also couldn't help but notice that Sam's penis stayed flaccid the entire time the two of you were fucking, even when came. As if some mental switch had already gone off to tell him that he was a woman well before his body had changed.