I'll not go heavily into the shoulds and shouldn'ts issue except to say that I personally basically agree with a permissive approach between freely consenting adult individuals so long as no permanent physical harm is inflicted. His thought however is that the laws as written do give the states to regulate it. I suspect from a legal standpoint, that he may be correct.
Most laws are rather general. If we do not declare adultery to be illegal, then how do we use it as a basis for dissolving a marriage contract? Do we permit and accept bestiality? Do we define the age of consent very rigidly, such that an act between teenagers on one weekend is illegal and the same act a day later is fully permitted because someone had a birthday? If not, how do we protect minors?
It does open a very tough area. When does the good of society override the right of personal privacy, especially in matters of a sexual nature?
The only thing I can think of that may be more difficult is once you've decided, how do you word it?
If you are interested, the text of his original remarks are here: http://www.cnn.com/2003/ALLPOLITICS/04/22/santorum.excerpts.ap/index.html