I didn't state any of my own personal opinions. What's honestly awful are people who try to pretend everything is an opinion and that all discussion must end "let's agree to disagree". Which means the conversation as a whole was pointless.

Anyway, you clearly have very selective vision and block out things that you know damage your stance. For one, you ignored the whole point about her monologue and her saying she was actively trying to hurt Sono. Another thing you ignored is the word shallow. Which is what sleeping around is, in every objective meaning of the words. You're right, people are different, and by the same note, some people are worse than others. Not everyone is equal, you don't have inherent worth just because you're a human. People who think it's fine to sleep around are statistically proven to always cheat when they even try to end up in committed relationship.

Wanting to sleep around literally means you are less evolved.
https://www.google.com/search?q=genetic+predisposition+to+cheating&oq=genetic&aqs=chrome.0.69i59j69i57j69i65j0l3.4651j0j4&sourceid=chrome&ie=UTF-8#q=unfaithfulness+genetic
The impulse sleep around and cheat comes from a primitive animal gene to procreate the species. Obviously since in an evolved world the need to procreate the species is never an issue, the gene to sleep around is no longer necessary.

Also, of course, if you sleep around, your are openly saying that you do not want to and are not willing to devote yourself to a single other person, and as such your feelings for them are inherently weak. Even if they do end up in a committed relationship, they still slept with all those other people, those don't just go away. You are literally used goods. So what? Even if you find a partner you are to assume they just accept your post and that you slept around like a slut? Assume they do accept it, they just settled for less when they could have had an actual good person as their partner. Mathematically not a fair situation.

And you're right, a lot of steady relationships do break up. Because there are obviously more bad people than good people. If being good was the norm, it wouldn't be good, it would be the norm. In order for good people to exist bad ones must as well. Sleeping around and being shallow/depraved means you are not not faithful and obviously are the type of person with shallow feelings. There's a reason over 95% of America says it's a bad thing. While ad populum is obviously not a valid argument, I've already explained the reasons why that 95% thinks that.

Now, back to the manga. The friends jokes about her being terrible, because guess what? They don't think it's a big deal because they are terrible people, too. As evident by how one them tried to fuck Sato right away with no real reason. They're like you, they think being terrible people is fine. And like I pointed out before, you totally ignored the part about how Hiroko had every intention of trying to hurt Sono.