Are you also worried that your little boy will see some woman's huge breasts in a bikini as well? Yeah, right! This is just part of the bullshit and false worry about the human body that insecure parents teach their children and they pass it in turn to their kids. It's this insecurity that keeps parents from teaching their kids about sex & equipping them for their sexual health & safety. It's also about helping your kids establish better self esteem about their bodies.
Go to Europe and you'll see adults & children exposed to just about anything as natural. Look at their statistics and you'll see far lower rates of rape & other sex crimes, teenage pregnancy, body appearance problems (anorexia, bulimia,) STDs, etc. They are by far better adjusted than most Americans when it comes to sex and happiness. It's the same for most of the rest of the world. We unfortunately got fucked up by our Puritan societal roots.
I'm not suggesting that you run out and expose your children to sexual acts or sexual situations, but the natural human body itself is not inappropriate for children. It's only natural for children to ask questions when they see things. It's a parent's responsibility to give honest, age-appropriate answers.
I would be far more worried about what my kid sees on TV or in school, than some man or woman showing a wet outline of a body part on the beach or at a pool!
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Now back to the underwear thing. Like most young men's clothing trends, I think it has a lot to do with sexuality, conformity, body image, and insecurity. Having underwear sticking out is sexually suggestive by drawing the eye to the butt or crotch. It started as a prison-bitch thing that just caught on...
Ok. before this gets confused somewhere, I don't have any children, I'm just saying when I do. Ok, anyway...
No, I guess I'm not to worried about my son seeing someone's breasts, but that's a little different. It's hard to avoid something like that in everyday life. Seeing someone's penis bludge, pubes, asscrack, not so much.
I will have no problem teaching my son/daughter about the "birds and the bees", masturbation, and all that good stuff, but at the same time they should know NOT everyone is as comfortable with their/anyone else's body, and because of this, it's better to be conseritive, than walking around in a thong.
Also, I don't live in Europe, and have no plans to, so non of those things are worth me looking up. I believe you, but, I'll be blunt, I do not care. I live in America, where rape and such things are a worry. Also, while speaking in other countries, in Africa they barely wear any clothes, yet AIDs is all over there.
Over all, I'm not saying I don't "expose" myself for my children, but I'm sure there's women on the beach who share the same concern I do. When out in public, there are certin things that you do have to do that you would rather not. I might like to swim naked, but I'm not going to jump in the beach naked because I'm sure there's some people out there who wouldn't want to see my bare ass. (Tragic, isn't?)
Bottom line, I'm comfortable with my body, but not everyone is.