I, for one, have very little understanding about what an individual TSA agent can or can't do to me. All I know is, they tell me to do things and I have to do them, or I'll receive some sort of punishment I can't get out of. I presume Larson is in the same boat.

Being ignorant of the law and your rights is not an excuse to be in fear of anyone wearing a uniform. Regardless, we're not living in some corrupt draconian third-world country. Even if you are ignorant of the details of the law, if you're of average intelligence, you should have a reasonable idea of what constitutes an acceptable request and what doesn't. If you're in fear of being strip searched because you didn't give a TSA agent your number, that seems somewhat paranoid.

No; anybody in a position of power has the responsibility to not put people in a position where their power could be used to intimidate or manipulate someone sexually. She is in a vulnerable position compared to him, and any request he makes is therefore something she can't respond to in a totally free way. ... maybe he wouldn't punish her! But he COULD. And he has a responsibility to not put her into that position.

Absolutely, and that's a good argument for why the TSA agent's action was inappropriate and unprofessional. But being in a position where you have the ability to sexually harass someone with impunity is different from actually sexually harassing someone.

The example from "It's Always Sunny" (IAS) is a faulty comparison: (i) the scene from IAS directly involves an invitation to sex, whereas asking someone for their phone number does not. (ii) In IAS, they are literally in the middle of nowhere; in Brie's situation, she's in an airport, surrounded by tonnes of people, and there's plenty of other TSA agents and other people in authority who could help her if need be.