Where to put CAUTION tags on multipart stories?

As an example, say there's a story of six chapters, and there's only sex in chapters three and six. Where do you put the CAUTION: Sex / Sexual Scenes tag?

Does it go on the organizer page? At the top of each chapter? Even on the chapters with no sex at all?

On the one hand, I try to err on the side of CAUTION, so that no reader will be unnecessarily shocked or disturbed by reading something they'd rather avoid.

On the other hand, I realize that some readers might be looking for exactly that tag, and be disappointed when the advertised sexual content isn't there.