Ryu is sleeping with his college professor, but doesn't feel loved, so he decides to kidnap his professor's son, Hayato, out of revenge. As it turns out, Hayato's relationship with his father is not very good, so he winds up living with Ryu.
The doujinshi called "Nezi no Kaiten - EX", (done by the mangaka) begins when they start living together. It was released inside the book -6mm no Taboo.

Ace, a human teenager, has been raised by King and Jay, vampires who have lived an eternal lifetime. As he grows up, he became attracted to Jay and Jay also comes to yearn strongly for Ace's blood. King and Jay, afraid that their secret will be exposed, decide to send Ace away to a boarding school. But when Ace finds that out, he wishes to be a vampire like them.
Sadahiro Mika's Gothic love story about the growth of a teenager who's raised up by vampires and the feeling of a vampire who live in eternal life.
(from the cover)

Also contains two additional oneshots:
1) Deep Throat (beware, this is also the name of the extra at the end of volume 1 of Pathos, but it's two different releases);
2) EX.

The person Aiba Misao loves is an onnagata in taishuu engeki, named Yumedono Ginnojou. He fell in love when he saw this person on stage, believing him to be a girl. Thus, when the plain Oosato Ginji is transferred to his school, he fails to recognize him as his much-admired Ginnojou. However, Ginnojou finds love, becomes even more enthralling and makes a big break in a commercial. A long-distance relationship, a frustrating love… Complications turn these two people into adults!
In addition to the title series, also included is a hard love story where East meets West on the streets of New York!
(Source: Nakama)

[From Nakama]

30 meters underground, a prison where escape is impossible; the United States penitentiary, “The Underground Hotel”. Only the people who enjoy living in this place, a life cut off from the sun, call it by another name: “The Under Grand Hotel”. Absorbed in pleasure-seeking sex, the shut-call, Erie (leader of the prisoners) isn’t watching the men he’s raping each night, but rather one man alone. Norman.

Norman, who keeps trying to harm himself as a result of trauma, the kind doctor who cares for him, Van, and Erie, who’s falling into a brutal love that’s more akin to hatred… What will happen when these various feelings are made to intertwine into a closed-off world? From storyteller Sadahiro Mika, the long-awaited, mature love story of the “Under Grand Hotel” begins!

Under Grand Hotel (UGH) is the inmate's name for a prison in the US.

Sen is a Japanese guy who has been imprisoned after killing his lover's husband. As expected in prison, there is always the fear of getting raped by multiple inmates. Unless he can secure the protection of a powerful inmate first, his ass belongs to everyone – literally! Aptly named Under Ground Hotel, this maximum security prison in situated entirely underground; but those who had come to rule it and love it, prefer to call it UNDER GRAND HOTEL.

This is without a doubt, the smuttiest manga you will ever read!

Sword Fish – the current prison boss is a dangerous man. Other inmates fear him, and take utmost care to be on his good side. What Sen doesn’t need to do is pick a fight with him the first day he arrives at the prison; yet, that is exactly what he does! What will happen to Sen now? Did the situation just get worse for him, or will he manage to rise out of it alive? Will there be somebody – just anybody, helping him in this volatile place or is he cursed to rot in here all by himself? And what of Swordfish – the prison boss with the deep cerulean blue eyes? What are his intentions for Sen? Is Sen doomed to rot in here for the rest of his life, or will he be one of those lucky ones who get to call this place UNDER GRAND HOTEL?

**Warning: very graphic and explicit***

Otoji is searching for his older brother in seedy, urban America. During this search he meets up with Joe, a fashionable prostitute who eventually joins him with his search. Along the way Joe aquaints him with the wonders of sex and prostitution, and the flexibility of being both seme and uke! But while Otoji deals with the search, including finding his older brother and sleeping with him, he and Joe face the darker side of their lifestyle.

Toma and Miyako, who are living life on the run, and Itsuki, a wealthy idler who shows abnormal interests. Is what lies beyond the abnormal sex unfolding day and night light or darkness...!? Love is a labyrinth, the other side of which is desire polluted with blood and semen. The concluding volume of Mika Sadahiro's full-spirited SM love that will change the world!!
(Source: Nakama)

Tom and Miyako were raised in the same orphanage. From the moment they met, Miyako has been unable to repress the impulse to soil Tom's beautiful hands. Sadistically embracing Tom is the proof of profoundly sweet love. What awaits the two who wound each other and lick each other's wounds!? The hard Eros of madness and pure love! The forefront of forbidden SM!
(Source: Nakama)