I don't know what juris-dick-ion is on penis at Portland International or airports in general but Oregon has no obscenity laws due to the way the Oregon constitution is written. Since the arrest was made by Portland police it seems to indicate that this falls under local laws. The Oregon constitution's free speech language is why Portland has naked runs and naked bike rides every year without arrests. See State of Oregon v. Henry
Unless they have evidence of "attempting to arouse sexual desire" this appears to be clearly protected under free speech under the Oregon constitution.
"Being naked in public in Portland is legal if it falls within the guidelines of ORS 163.465, which are included below.
ORS 163.465. Public indecency
(1) A person commits the crime of public indecency if while in, or in view of, a public place the person performs:
(a) An act of sexual intercourse;
(b) An act of deviate sexual intercourse; or
(c) An act of exposing the genitals of the person with the intent of arousing the sexual desire of the person or another person."