IANAL, but sexual abuse is handled at a state level for the most part. Many of the state laws, and feederal law follows title 18 of the US code section 2242 which states:
Section 2242. Sexual Abus
Whoever, in the special maritime and territorial jurisdiction of the United States or in a Federal prison, knowingly -
1. causes another person to engage in a sexual act by threatening or placing that other person in fear (other than by threatening or placing that other person in fear that any person will be subjected to death, serious bodily injury, or kidnapping); or
2. engages in a sexual act with another person if that other person is -
* (A) incapable of appraising the nature of the conduct; or
* (B) physically incapable of declining participation in, or communicating unwillingness to engage in, that sexual act; or attempts to do so, shall be fined under this title, imprisoned for not more than 20 years, or both.
So, if the person being screened can not say "I wish to leave the airport instead of being searched" (say someone that is mute, or does not speak english and no translator is avalable). That could fall under paragraph 2, section B.
Now, the "out" for the TSA can be found in 18 USC 2246, paragraph 3:
3 the term "sexual contact" means the intentional touching, either directly or through the clothing, of the genitalia, anus, groin, breast, inner thigh, or buttocks of any person with an intent to abuse, humiliate, harass, degrade, or arouse or gratify the sexual desire of any person;
If no one was being aroused, than it was not sexual contact, therefore not sexual assault. That is one LARGE loophole.