I don't think you understand how Copyrights work. In short when someone creates something and puts it on the internet they can legally decide what other people can do with it. In the case of porn, I'd say for most of it people are forbidden from sharing, you just need to read the copyright in the porn website. So basically the only people that can share porn are the ones that have the copyright. So, since you don't own the copyright, you are not legally allowed to share it.
But why only some get flagged? Because for something to be taken down there must be someone flagging it. Gaytorrent is a popular torrent website, so most likely it has more people keeping an eye on what's here. So it isn't that the other websites are allowed, it's that it's hard to keep track of stuff, so some things get flagged while others don't, some websites are more targeted while some aren't, some companies are better at flagging stuff while others aren't.