Set in a typical high-fantasy world, a fair maiden is "cursed" to become a man.
No longer recognized by her family and seeking asylum, she joins the royal guard. No one there knows her past, she was not pressed, as long as she swore allegiance to the king.
She keeps her nature hidden, or tries her best, but a higher-ranking knight finds out about her secret because she just doesn't know how to act like a man. The rookie panics because oh no, she's been found out! She'll be kicked out of the royal guard and have nowhere to go!
"Do not fret," he reassures her. "I can teach you everything you need to know."
And he does. Everything from how to talk, to how to walk, to how to fight, even... how to pleasure oneself with the rookie knight's new body. He comes embarrassingly quickly, but his superior assures him that he'll get better with practice.
It's incredible. He's never felt so alive, and he thanks his superior for all of the knowledge he's bestowed upon him. The rookie asks what he could possibly do to repay him.
And so now, the senior knight has his little pet rookie who sticks suspiciously close to him any time they're not training, and who gets special attention during. No one can find any proof that they're not just friends.
No one sees the rookie knight learning how to take a sword like a man, in the secrecy of his senior's quarters.