Every living creature discriminates. When a creature chooses a mate, it discriminates. When a teacher gives a grade, he discriminates. When an NFL team picks a draft choice, they're discriminating. When a restaraunt hangs a MEN'S sign on the restroom doors, they're discriminating. What makes it wrong/illegal/immoral/unjust is when the lawful authorities (to whom the victim pays involuntary taxes to help guarantee their own protection) use unjust reasons to discriminate against them (most commonly, the circumstances of one's birth - gender, race, etc).
There is nothing illegal, immoral, or unjust about me telling my daughter that she cannot bring home a violent felon, a leper, or even a gay man... and the law cannot force me to invite them into my home. The Boy Scout's logic, that it would be innapropriate to put an unrelated adult in charge of defenseless minors of a gender to which that adult is attracted, is neither illegal, immoral, unethical, or unsound. The exact same decision of a family, to not let his adult male neighbor host their well-developed 14 year old daughter's sleep-over party, is and purely discriminatory... and rational... and legal... and proper.