Families are of course entitled to their strongly held and detailed beliefs, and I applaud strongly held and detailed beliefs, but all you will get is statements, no reasons, as @Kaja 's posting points out. You won't get any whys.
Sometimes, even on the basis of statements, it is possible to find out what the reasons are behind them. Therefore, even these types of answers are valuable in my opinion, especially if they are sufficiently developed and detailed.
youdontknowme If you were to argue that female modesty is more important than male modesty because that is a tradition you identify with, I can accept that as a good argument for living that lifestyle yourself,
He is saying it. He does not expect convince to anyone. He responded according our thinking:
They are our beliefs, not science.
Kaja But it is a CRIME that you are DESTROYING sexuality of your childrens.
He can respond by himself, but firstly in our family any child wears a belt, and anyone has sexuality destroyed.
Modern medicine and psychology adds the caveat that masturbation is a problem if it becomes distressing to the masturbator.
I have never come across such a reservation in the context of masturbation, rather in cases of sexual paraphilias. It is possible, that some doctors and psychologists use an analogy here.
Further, society only accepts masturbation is normal if it's done in secret.
It's rather obvious, as with sexual relations.
The NoFAP movement and the widespread availability of hardcore porn has had a effect on modern medical opinion. There is a noticeable push-back on the relaxed, liberal, sexual views of the 60s onwards.
Looking at the changes in the ICD classification in recent years (ICD-10 -> ICD-11), I would say the exact opposite.
Andrew So I wonder what makes them reject the view expressed by mainstream medicine and psychology.
Truly there is not "mainstream" medicine talking about It. There are some articles, but mainly related to male masturbation, there are not medical evidences about any benefits of female masturbation.
Relief sexual tension, strength vaginal area, sleep well... are not benefits, since they are small, exist other ways to achieve those, or are unnecessary in most of girls.
Sexology is the "science" that touches more the topic, and obviously it has an important basis bias.
Andrew but none of them apply here...
No, certainly there is better motivation an reasons.
I share most of thinking of my uncle, but I think that exist another reason, it reinforces the path of behavior, setting and accomplishing the prohibition, it is obvious that all bad behaviors of a sexual nature move away from us.
I am not saying that this is nothing set in stone, and there are girls like @Sin or @Kaja who have another philosophy ans it's perfect for me, I do not say my morality is better, but I do not want their life for myself.
Jonas Further, society only accepts masturbation is normal if it's done in secret.
Private, methinks, is how it should be done. If you force your children to keep their sexuality a secret they are apt to feel it's something to feel guilty about. They will likely feel they cannot discuss it with you and will get their information from other people which is not what many folks here want.
Kaja Im sorry for you.
I feel exactly the same for you.
I find very disgusting the apology of sexuality in your environment. 
I am sure you do not want offend to anyone, as usual, for that your carefully selected words
Kaja your parents use brainwashing
Yes... because a young girl spoiled by sex addition is much better than a girl behaving as is supposed she has... Bless brainwashing, then.
I am grateful everyday of my life for my brainwashing.
Addressing the original topic, I do not see masturbation as wrong. It is an activity that does little if any harm to others or their property. It can I suppose, like many other human activities, cause harm to the person doing it if they do it to the point where other activities of their lives are impacted, though I suspect this risk is overrated. Most activities done to excess can be harmful, to a greater or lesser degree. Masturbation is one more. I do think if there are partners, then the partner under certain circumstances could be impacted in a negatively emotional way.
In general, masturbation impacts no one other than the masturbator, and thus nobody else's opinion should matter. I say in general because one can always thing of extreme cases - for instance I would not be too keen on the pilot of the airplane as was in, masturbating instead of flying the damned plane! 
Ines there are not medical evidences about any benefits of female masturbation.
Then there is certainly no medical evidence of a negative influence or effect when women masturbate.
Ines Relief sexual tension, strength vaginal area, sleep well... are not benefits, since they are small, exist other ways to achieve those, or are unnecessary in most of girls
What speaks in favour of this is the beautiful feeling, which is not achieved by other means.
Ines I share most of thinking of my uncle, but I think that exist another reason, it reinforces the path of behavior, setting and accomplishing the prohibition, it is obvious that all bad behaviors of a sexual nature move away from us.
Since it feels right for you, that's what you should do. I think that's good.
Kaja your parents use brainwashing.
I don't think so. I am convinced that @Ines is very capable of forming an independent opinion.
I was also brought up as a Christian. Nevertheless, I came to the conclusion that there is no God and I am now an atheist. Education may have some influence, but it doesn't have to.
Foxies for instance I would not be too keen on the pilot of the airplane as was in, masturbating instead of flying the damned plane!
Either way he will have his hands on the stick.
Andrew It seems to me that the main benefit in this case is the pleasure derived from stimulation and satisfaction of sexual drive ...
No doubt. Also I've found, before I stopped doing it, at least two other benefits which helped me a lot:
Your mileage may vary.
Ines Exactly.
I found a link to a study entitled : "The Role of Masturbation in Healthy Sexual Development". The title implies, I think, that masturbation contributes to healthy sexual development. But reading the study it's more a chronicle of problems: guilt, lack of information, being called a sissy, etc. I'm pretty sex positive but it was interesting reading about all the difficulties many young people had.
