Hey, how do you know that it was marked as spam? Do you have a source for any of that? People will pay attention if you do.

There's something called the "minus thirty" penalty which is applied to some spam sites, and these sites are displaying exactly that problem - thus, it's virtually certain it's a bug in the algorithm which determines which sites are spammy, rather than anything specifically targetted to those sites. Other, non-sex, sites have complained, there's some threads discussing this on SEO forums.

I could have spent time gathering references and writing this up, then even more time going around to the various echoes and trying to get them to pay attention to a obscure blogger that says their too-good-to-check ("porn clampdown"!) story is wrong, but it's just not worth it.

http://www.comstockfilms.com/blog/tony/2006/12/28/google-when-language-fails/

How often can this bug occur. This is the second time around....

Repeatedly. See my post just after this one. It's like asking "How often can a mail server's anti-spam algorithm yield a false-positive"? Especially e.g. concerning a legitimate medical newsletter for people who take the medication rhymes-with-Niagra.