since the Catholic Church makes no dictate as to the actions of non-Catholics and in any case they would not obey her even if she tried, why would you even ask this? If artificial contraception is harmful and sinful in its very nature, it is harmful and sinful because it violates natural law, a law God put in place, not the Catholic Church. If it has harmful effects, those effects will ensue no matter who uses it, no matter why, and regardless of what any church says or does not say.
That is like asking why, if the Catholic Church says murder is a sin, she is not violating rights of non-Catholics in some way. Someone who is murdered is still dead, and there still is harm to society no matter what the church says about it.