You and I are going to disagree here.
You are correct that the state of the US is one that many things cannot be done with the law. But in the ultimate sense, the laws should always be conforming themselves towards the ultimate good. Laws which are removed from this, or are only in proximity towards the good, are also that much unjust.
Catholics are still obligated to do the good regardless of how the world sees it. A Catholic should not support legalizing activity which violates the dignity of human health and sexuality. Its not about what the state thinks is right, or even what society thinks is right.
Mankind is lost without faith. Politics should always be in service to the truth.
The United States has only been around a little over 200 years. In that time, many groups have imposed their moral view upon the populous, including socialists and communists.
Why should we be any different?
RE: A Catholic should not support legalizing activity which violates the dignity of human health and sexuality.
Then I gather you support government criminalizing immoral acts like sex between unmarried consenting adults in the United States?
This is LEGAL in the USA you know.