To admit that sex should not be mentioned is in saying that it is essentially bad that a child even knows of sex.

No one's saying sex should not be mentioned, unless I'm missing something. What dr_jekyl is saying, as far as I can see, is that we'll talk about sex all we like thank you very much, but first we're going to put up this sign saying "sex discussed here".

There's no shame in that, and it's an entirely true statement. Concerned!soccermom can look at the sign and determine it's not somewhere she wants little Johnny to go - and whether or not I agree with her belief sex is a badbadwrong thing for her child to talk about, I'll not deny her the right to determine that for herself and her own family.