Law enforcement officers and prosecutors investigating or prosecuting indecent exposure cases should be aware of the following statutory elements and judicial interpretations of Iowa's law.
Indecent exposure is prohibited by Iowa Code § 709.9, which provides:

A person who exposes the person's genitals or pubes to another not the person's spouse, or who commits a sex act in the presence of or view of a third person, commits a serious misdemeanor, if: (1). The person does so to arouse or satisfy the sexual desires of either party; and (2) The person knows or reasonably should know that the act is offensive to the viewer. 

In order to prove this offense, the State must establish the following four elements:
1. The defendant exposed his or her genitals or pubes or committed a sex act in the presence or view of a third person;
2. The defendant did so with the specific intent to arouse or satisfy his or her sexual desire or those of a victim.
3.  The victim was offended by the defendant's conduct.
4. The defendant knew or reasonably should have known that act was offensive to the victim.
Iowa Criminal Jury Instruction 900.5

The Iowa Supreme Court interpreted this statute twice in the 1980s.  In State v. Bauer, 337 N.W.2d 209 (Iowa 1983) the Court held that section 709.9 was not unconstitutionally vague on its face because of the third and fourth elements of the crime, i.e., that the State is required to show the state of mind of both the actor and the victim-viewer.  The latter must be actually offended by the conduct and the former must know or, under the circumstances, should know the viewer would be offended. 
The sufficiency of the State's evidence for an indecent exposure prosecution was reviewed in State v. Adams, 436 N.W.2d 49 (Iowa 1989).  The Court reversed a conviction finding insufficient evidence for an indecent exposure when a child victim saw a man exposing himself on April 14^th but could not identify him, and only a police officer saw the man exposing himself on April 15^th when the perpetrator could be identified.  Although not expressly holding so, Adams suggests that a police officer may not be the victim for purposes of the indecent exposure conviction.
Law enforcement considering a sting operation to deter indecent exposure in public places must be cautious not to invite the "offensive" conduct if the goal is to prosecute the subjects of the sting.  If officers appear to be willing participants in the encounter, it will be difficult for prosecutors to show the defendant knew or had reason to know the contact would be offensive.  The safest route to successful prosecution of these cases would be to seek testimony from citizens actually offended by unwillingly viewing such conduct.