Let me rephrase that: if they pose an immediate threat to people. Gluttony is not an immediate threat the way murder is, and people can change their minds about it and quit. It’s unlikely that a killer is going to change his mind in the middle of a gunfight or right before an abortion. Also, gluttony is something done in your private home, where the govt has no business. Murder and abortion and gay unions and all the others are done on the public scale, where the govt does have influence.
Also the govt has the choice of authorizing funding to things like stem cell research. They are in the position to ban it, and should. They are not, however, in the position to ban gluttony.
While I don't like the idea, I'm not seeing gay civil unions posing an immediate threat to people. You're opening a door to a standard for ignoring the Constituion and the separation of authority that's entirely too subjective to suit me.