The ACLU says so and I more or less agree with them. In an amicus brief filed with the Minnesota Court of Appeals, the ACLU makes the point that a law that seeks to punish the mere invitation to sex is overly broad and violates the First Amendment. After all, if one walks up to an adult and says, "hey, let's do the nasty," one has hardly violated anyone else's rights.
In contrast, if the airport had a "no flirtation in the bathroom" policy, they would have been within their rights to eject Craig from their facility--but that's a civil issue, not a criminal one.
But perhaps most of all, I simply love that the ACLU is defending Craig along privacy lines-lines that most conservative Republicans are loath to defend. It puts Craig in an awkward position because if he wins on the basis of the ACLU's arguments (and not his own "wide stance" argument) he's going to be the Republican Senator who got out of a public lewdness charge because the courts ruled that he had an expectation of privacy while tapping himself away in the airport men's room. And as we all know, if there is one thing the world likes, it's the taste of sweet delicious irony.