There are only two important categories.
Futanari = TRAPS :shock:
Futakoi = TWINCEST 8) … or rather it used to until a certain anime used that as a title. Seems that mundane 'ole “Futago” is the word of choice… probably because Futage might give people the wrong idea (see: TRAPS).
In the context of adult entertainment, guro is the category of ero which encompasses what “normal society” considers grotesque and disgusting. It’s a broad spectrum term that covers everything that’s visually horrifying: scat, urination, dismemberment, gore, disfigurement, etc. Guro does not mean rape per se; although rape can be guro. For example someone forcing a girl to have sex with them, and then cattle prodding her with electricity until she wets and craps on herself from the pain. That would be guro. So would be ripping her arms off, cutting off her head, or just mutilating her body with permanent disfigurement.
We still on the same page? Now here’s where it gets complicated.
As the years went by, people began to deconstruct guro into subcategories. Making it more distinct than just “one word to cover it all.” Today most erogers will say guro is ONLY the violent stuff that has people being torn apart with blood and guts splattering everywhere. For example tentacle rape USED to be classified as guro, but now it’s classified only as tentacle rape, unless the tentacles are tearing the heads and arms off girls, or bursting out their wombs in gore¬Ö then it’s “guro tentacle rape.” Ero with scat USE to be classified as guro, but now it’s only called scat, unless the person’s intestines are being ripped out and openly exposed into the open.
If you see internal body organs because someone violently tore them out and threw them in the open: 90% chance it’s guro. 
One final note: Guro has a “special” sub-category called Vore. Vore is when something is eating and digesting someone… with all of it shown. For example a werewolf eating the arms off a woman while he’s having sex with her. It might be a giant venus flytrap that a person fell into, and showing how the acid slowly burns away their skin. A title can be guro and not vore, but a vore title is nearly always guro (cause they tend to show the gore).
Guro are popular because of the shock factor. Sorta like bloody shasher horror movies. There are guro titles with great stories and plots. Black Cyc used to be my favorite when it came to these kinds of eroge - they’ve kinda lost their touch and “toned down” the violence as of late unfortunately. Nonetheless, MinDeaD BlooD and Gore Screaming Show are great examples of guro. There are people who generally dislike guro, but enjoyed these titles.
As someone stated earlier, guro is not only limited to sex. Over the top horror movies can be guro: Hostel is a good example. So are Texas Chainsaw Massacre flicks. I’ve seen some even say, medical video recordings of organ transplants used for trainning and professional review are guro.
), but I do not know/remember if a specific category for “fetish/??? games” exists…