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A few basic links from adult sites will not affect your rankings in any way, except that your PR may benefit slightly which might have a marginal affect on SERPS.
There are many causes of pages being dropped.
1) Take a random sampling of a few pages that have been dropped and scrutinize them carefully (source code). Look for validation errors, bad links, empty links, hidden text, etc. Correct any mistakes. Don't assume that pages that are still indexed are faultless.
2) Google page-hijacking. It's possible that you're a victim of this but don't jump to this conclusion.
3) Look through old threads - there are many on this subject.
Also, check and validate your robots.txt file - it's not a likely cause but this only takes a moment so do it anyway.