Stephanie Slade, a libertarian Catholic, defends fusionism in Reason. There are, she argues, both practical and moral reasons for believers in the basic moral obligations entailed by “the Judeo-Christian tradition” to deny that it is “the state’s job to enforce those moral obligations.” Both the danger of tyranny and the dignity of the individual suggest that we should tightly limit government. “Only when absolutely necessary—say, to stop one person from initiating violence against another—is it morally justifiable to overrule someone’s right to live his life as he chooses.”
All of this makes sense, up to a point. It isn’t the state’s job to enforce all moral rules, there are good practical and moral reasons to limit government, we should not limit anyone’s freedom without good reason. It does not, however, follow that respecting human dignity requires never allowing the government to foster a healthful moral ecology and never allowing limits on liberty toward that end. If you’re a reader who thinks prostitution should be legal, ask yourself: Do you really think that limiting billboards for it, or putting zoning restrictions on it, are an impermissible attack on the dignity of the individual?
The main problem with the way fusionism was historically elaborated was that its mistakenly categorical premises precluded the commonsense answer to that question. That “up to a point” matters a great deal, and covers a lot of legitimate debate that makes up much of the substance of politics.