Emily, I'm not saying you are one of the "force people into believing" folks. I thought it would be helpful to point out the consequences of going down this particular idea road, however.
In the past, it was not law that kept people from committing suicide, performing homosexual acts nor contracepting. The laws existed, yes, and were patently unenforceable. But such behavior was ultimately prevented by the overwhelming understanding by the individuals who comprise society that these actions were grossly immoral. One should also note that when the laws prohibiting these practices were finally lifted, these actions had been accepted, almost openly, for the decade preceding the repeal of the laws.
Many laws are only enacted when society as a whole recognizes something as a manifest evil. It's why slavery wasn't abolished in 1791. The public sentiment has to be fully behind it. And public sentiment can't be imposed by a law.
You cannot objectively quantify or prevent "an intrinsically evil act against nature" in itself. You have to quantify empirical acts that have been of detriment to non-consenting individuals. In this case, the two parties have consented to an action, evil as it may be.
What is the "common good?" Is it not the sumtotal of the good of each individual in society? Now, you can take "Common good" in a spiritual or a physical sense. Laws exist to uphold the physical "common good." The Church exists to uphold (not through violence, like the state upholds its laws) the spiritual "common good," through teaching and good example.
The "community" does not have any new or special rights, or more rights than each and every individual in society has. I do not have a human right to not have homosexuals performing unnatural acts two doors down. I do have a human right to my property and bodily integrity. If spiritual integrity is upheld, it can only be through protecting individual property and individual human rights. Otherwise, you need virtue police. How can you have laws against homosexual behavior unless you are knocking on bedroom doors? An unenforceable law is not a law at all, I'm sorry. No matter what natural law such a civil law proposes to uphold.
If someone is committing a purely spiritual/natural evil that does not violate the will against another person, nor quantifiably harm another person, how can you punish it? (Remember, laws backed up with force can only exist to uphold the rights of men, not the rights of God.) The Jews had such rules imposed by God. You were also free to leave the Jewish faith whenever you wanted. If every country in the world had such laws, would we be morally able to leave the world?
When it comes to pornography, #1 U.S. industry as it is, the only way to get people away from it is convincing them of why it is evil. We had laws against it before, and it was unenforceable then. The only reason it was not the biggest industry then was because people recognized why it was evil and abstained from it. Changing someone's mind has to come from convincing, not force. Otherwise, it ultimately boils down to "Why shouldn't I do this?" "Because it's illegal." "Oh. Well that's not a very good reason. It's a penal law. Great."
Prostitution also violates "intrinsic human dignity." Both St. Augustine and St. Thomas said that prostitution should be tolerated, for fear that more and greater evils would ensue. I believe the unending "war on drugs" is similar. If not for the artificial shortage of these chemicals that drives prices up and makes illicit drug dealing lucrative, there would not be such widespread turf wars and violence, not only by dealers, but their users who rob and kill to get money to buy it.
Prohibition is similar. They were passing a law ultimately intending to curtail the abuse of alcohol. A good end, yes? Alcohol abuse actually went up during prohibition. Sts. Augustine and Thomas were right: you need to allow certain evils in order to prevent greater evils. That is why God allows moral evils on earth: because it would be a greater evil to crush our free will.