This response doesn't strike me as either useful or fair. If the person who asked the question is willing to read several chapters of a story before giving up on finding a sex scene, that implies that this reader likes some buildup, too, and probably wouldn't be too interested in a PWP story.
Agreed. You mods are busy enough with serious issues (dealing with plagiarists, preventing lawsuits, keeping out the twelve-year-olds).
I think it's fair to assume that if a story has no tags whatsoever (official or unofficial), it's likely to be devoid of sex scenes. That, or the author's attention to detail is poor enough that their writing might not be fun to read, anyway.