In our society, we have defined "having sex with someone who has ever had sex with another person" as a normal behavior--yes, even devout Catholics believe it is fine to marry a late convert, or a widower. When 50% of the population has had the disease in question, and it is transmitted by activities that we consider a normal part of life, it's within the realm of public health.
This article available online at:
http://www.theatlantic.com/national/archive/2011/09/how-can-a-libertarian-be-in-favor-of-mandatory-vaccination-but-not-mandatory-health-insurance/245096/
