This immensely popular h-series has been said to have changed the history of adult animation with it’s scenes of violence, perversion and lust, filled with hardcore sex and unusual sexual fetishes! It consists of three different anime titles each with their own series of releases; Isaku, Shisaku and Kisaku. They are named after the three sadistic brothers whom they are about.

Is it ok to listen to anime songs?

Yes it is! I’m an otaku and listen to anime related songs all the time. Anime openings? That's absolutely fine.
Some aren’t that good, in my opinion though.
Except….Unravel (Tokyo Ghoul Opening) (My fav) I also got one of my friends into anime and we memorized all the Japanese lyrics.
Which sounds kind of weabooish, but that was only because that one song was so good.
Anyways, it's completely fine.
A lot of people feel kind of iffy because anime is viewed as weird and nerdy, but if you really like a song, no matter what it is, listen to it! Vocaloid? Vocaloid isn't an anime, but the vocaloids are in an anime style and are Japanese.
My favotites are Luka Megurine, Len Kagamine, and Hatsune Miku btw.
It’s completely fine to listen to them.
Thousands of people listen to it! I personally never sing these, but I like to sometimes listen to them.
Anime is viewed differently from people who don't watch it, and I don't think some ppl will ever understand it.
But if you do watch it, its fine! As long as you enjoy it! Also at first, I didn't really listen to or like it, but now I do! Hope this helps!