If they pose a threat to people and to society, they’re “big” sins. Thus, gay marriage, abortion, murder, embryonic stem cell research, cloning, and euthanasia out to be banned.
By the way, are we arguing over whether it should be banned period, or whether it should be banned at the state or federal level? Because if it’s the latter, the states DO have a role in the amendment process, so banning it at the federal level would not ignore the states (see Tailgunner Joe’s post).
out=ought
In that case, there’s no need to present an argument that the need to enforce morality overrides the constitution.
The question was who gets to decide whether they're "big" sins, not on what basis you decide they are. Personally, I'd say sloth and gluttony can pose a threat to people and society, but I don't think the federal government was intended or authorized to exercise control over those things.