How is the topic of "well people are okay with this totally separate usage" such an issue? Obviously how an image is used matters. If you take a photo with a camera that automatically deletes it immediately, nobody cares. It's not the act of taking a photo that people care about, it's what you do with the picture afterwards.

That's why consent is great. One person might think the photo of them in a bathing suit that you want to put on your instagram is creepy and another might be fine. Letting the subject decide is a great way to avoid the issue.