In Indiana and Tennessee, the crime of public nudity is defined to include (emphasis added)

the showing of the human male or female genitals, pubic area, or [Indiana only:] buttocks with less than a fully opaque covering, the showing of the female breast with less than a fully opaque covering of any part of the nipple, or the showing of covered male genitals in a discernibly turgid state.

So if you're in a public place and find yourself with an erection, you are legally obligated to sit down until it goes away.

The statutes are Indiana Code 35-45-4-1 and Tennessee Code Ann. 7-51-1114. Mississippi law (Miss. Code 19-5-103) also authorizes some counties to enact a similar prohibition; there's currently a bill pending in the legislature (House Bill No. 1480) that would extend the authorization to all counties.