I think the advent of Tinder and other apps have made it easier to meet multiple people at once, so the issue comes up more now.
In the olden days (also known as the year 2000), you met someone in the pub, went out a couple of times, and then one of two things happened first, either you slept together, or introduced your girlfriend to your mates and that was that, you were exclusive.
Now, you're on Tinder, you've got 3 conversations on the go, and you arrange to meet up with all of them over a 2 week period. You really like Option 1, so arrange to meet up with them again. Option 2 turns out to be a disaster, so they're out of the equation, but you may as well just meet Option 3, just in case Option 1 ghosts you.
Option 3 turns out to be great too, the conversation was better with option 1 but by god option 3 is hot. You've got another date with Option 1, if that goes well then you'll show Option 3 the door. But then Option 3 asks if you fancy a drink tonight. You do, and you end up sleeping together. It's really good, but you think you still like Option 1 more.
But what if you drop Option 3 and then Option 1 turns out to be crap in bed, you've ruined what might be a good thing for something that might fizzle out. So you dangle Option 3 for a bit until you've slept with Option 1, and then you can make a decision. You were right about Option 1, he's definitely the one for you, so you let down Option 3 gently, but then Option 1 ghosts you and you're soundly back at square 1.
This is why conversations need to be so much more explicit now. Are you still on apps, are you dating with anyone else, sleeping with anyone else. And then you set your boundaries based on that.