The sharingan is an ability that is passed down the Uchiha family bloodline and grants several abilities to its user. It can copy the techniques of an opponent perfectly, allowing the user to steal their abilities. It can also predict the movements of a person, providing a limited form of reading the future. Finally, it can see through illusions, although the success of this would depend on the user's skill.
I am Madara Uchiha of the Uchiha clan, one of the oldest families in the village of Incognito. The sharingan has made us one of the most powerful and influential families not just in the village but the entire country. It is passed down the male genetic line and has secured our dominance. I would like to keep this ability exclusive to our gene pool, and must constantly fight against the greatest threat to our power: horny young teenagers.
It is difficult to prevent teens and other young people from "spreading their royal oats" and passing our abilities to competing clans. Expecting them to practice abstinence is like expecting a silly childhood hobby such as comic books to make successful transitions into movies: it's too unrealistic, and I would be laughed at for trying. Encouraging safe sex is unreliable at best in the pre-modern world. Condoms break easily, and people rarely use them when caught in the heat of the moment. Other clans competing against us would love to have the sharingan in their possession, and would aim to encourage this and actively seduce our members.
How can I protect our birthright and prevent this from happening?