In my mind, Tezuka is asexual, so here's something exploring that..
He was well aware he didn't feel sexual attraction -that he couldn't feel it- no matter how hard he tried. Watching Ryoma pull off his sweat soaked shirt, his shorts hanging obscenely low on his hips, didn't make his mouth dry or palms sweat. He knew that Ryoma was attractive- you had to be blind to miss that- that his pale skin seemed to glow against the shocking orange of the clay court. But he didn't feel the desire to run to him, to push him into the clay and have his way with him.
Instead his desire was focused on the other side of the court, to feel Ryoma's tennis, to fight back against that overwhelming force- to clash against it. Tailored with it was the need to kiss Ryoma under the French sun, to hold him close, to let him know that he needed him, that he loved him with such intensity that he didn't think he could ever put it into words. That, his lack of sexual desire didn't diminish any of this.
Ryoma turned and caught his eye, giving him the brightest little smile. And, for a moment, all of Tezuka Kunimitsu's insecurities melted away.