It's more that when you have sex you're vulnerable, not just physically but emotionally, you do it with someone you love and trust therefore. That doesn't mean it's ridiculous though. It's not humiliating. If my partner gives me a hickey on the neck I won't cover it up. If people looking at me can tell I've had sex - who fucking cares? People have sex all the time. Porn is the most common thing on the internet. Advertising and movies have sex in them. It's not a secret.
I don't care whether people know I have sex. I've told friends what I do sexually. But it doesn't mean that the privateness of my intimate experience was tarnished, because it was shared between just me and my partner. People can know what the sex acts were, but no one else will experience the emotions that my partner and I shared, because that was what was special.
As for why don't people have sex in public - the other side is that our society has a religious history, so social norms are pretty based on religion even when we're "secular". It's not because sex is inherently humiliating. It's rather, the concept of sex being humiliating is a social construct with roots mostly in religion.
If someone released photos/videos of me having sex without my consent, that would be a violation of privacy, but it wouldn't make ME feel ashamed to have had sex, it's more on the person who did it, says worse about them than it does me.
And if my partner said to me "I could tell the world I made you scream when you orgasmed, that you enjoy sucking my cock" I would be like - so what? I'm not ashamed of that. Most people in the world have sex, most people who have sex enjoy it, I am certain I don't do anything uniquely weird during sex.
My intimacy with my partner isn't due to blackmail (again, says something pretty sad about the state of your love life that you think intimacy is due to blackmail), it's due to trust and mutual vulnerability of EMOTIONS.