blackmail is an issue only if society and you care. if nobody cares it is kind of hard to blackmail
In America the rules on sex are simple - no minors, no force, no animals (
)
As far as falling birthrate in USA there are other issues in addition to sexual morality - economic stress on lower skilled males, child support requirements, bad parenting etc. We need better social support for divorced families and better education on personal finance. the biggest reason for divorce is incompatibility on finance not sex in USA.
It is a fair question how much coercion was involved