It is being sexually active where they disagree with the Church. They see neither a casual hook-up or a live in relationship as wrong, nor masturbation or pornography. The clerical obsession with sex and the need to confess them simply seem backward to them. If the Church would treat these matters as inadvisable rather than evil  (raping drunk girls-being the exception) because they complicate life and go against the natural inclination to more permanent connection, it would have better luck. Even St. Paul condemned fornication as a sin against oneself, not as an insult to God, who cannot be insulted. Only the pietous are insulted. They need to get over it or people will continue to stop coming.