But those laws by definition would imply enforcement. Enforcement would require the government to be allowed to monitor. Monitoring is certainly a privacy issue.
GRISWOLD v. CONNECTICUT, 381 U.S. 479 (1965) has some very useful linkages to multiple Constitutional amendments regarding privacy. How will the state know, unless it is free to invade one's home?
Note: I would argue that the state has the right to discuss public sexual behavior, and any private sexual behavior with minors. I'd be interested learning how to do that based on the Constitution, but I think it's easy: minors have the right to pursue life, liberty, and happiness and sexual abuse would clearly interfere.