One popular response to people worried about the scanners is to say "Come on, you're not that hot, no one's going to save your picture. No one wants to look at you naked anyway."
Leaving aside everything else wrong with this -- come on, haven't you seen enough of the internet to know better? It's just as likely -- probably more likely -- for someone to start a collection of "ugly," "fat," or "freaks" scanner images. Think People of Walmart, except naked. And most people, even people who look pretty normal on the outside, could potentially end up looking really weird on the scanner.
Another popular line is to say "Why are you so ashamed of your body? It's just a body. Why should you be so embarrassed if someone else sees it?"
Privacy is not the same thing as shame. Being comfortable with your body doesn't mean you give away all control over it. In fact, having control over your own body is a huge part of being comfortable with it. If you want to let someone to see or touch your body, you can. If you don't like the way someone's going to look at you or touch you, you don't have to let them -- because it's your body.
The scanners and pat-downs take that control away. It's not about worrying whether the TSA will like your body. It's about the TSA saying that while you're at the security checkpoint, you have absolutely no say about who gets to look at your body, who gets to touch your body, or how they do those things.
The only other places this happens: when someone is arrested, and when someone is in prison. In both those cases, the primary intent may be to find weapons, but a major secondary effect is asserting control over the person -- and police and guards know that.
Also: Most people reading this will be cis. Consider for a moment people who are trans, whose private parts may look or feel significantly different than the TSO expects. Both the scanner and the pat-down will out these passengers to the TSOs. A pat-down done in public could potentially out these passengers to everyone else in line, too, depending on what the TSO says or does.
Now consider how many people become angry, abusive, even physically violent, towards someone who's outed as trans. There's more at stake than just embarrassment.