This is always a question for anyone writing erotica. Just how dirty do you make your book or story? Do you try to keep it tame enough to appear in the bigger markets? Or do you go into areas that a lot of people think of as perversions, limiting sales to the handful of outlets that sell that sort of thing?

I do both. Of my four currently-published books, three are in the specialist category, and one is more basic erotica. There are three more books at the publisher now, which should be on sale soon, again in the specialist area. By specialist, I mean there’s some adult incest involved. And there might be a brief mention of a horse somewhere.
Honestly, this is more a marketing decision than a literary one. Erotica sells, and for some reason the kinkier types sell better than the tamer variety. Back before Amazon started cracking down, if you published essentially identical books, but in one you made the characters unrelated, and in the other you made them siblings, the second one would sell in the hundreds of copies in the same time it took the other to sell a couple dozen.
That’s one of the dirty little secrets of the erotica genre. A very large proportion of incest erotica is simply non-incest stories with a relationship added. If you have a guy in a threesome with a pair of bisexual waitresses, you gain some extra sales if you make them sisters. That sort of thing. School friends become siblings, an older neighbor becomes an uncle or a father. It doesn’t take much.

Amazon, of course, doesn’t take any of it. Not if they notice it, which they eventually do. One anthology I was a part of, though not using this pen name, was a perennial favorite on Amazon for several years, until someone noticed that a lot of the characters were related to each other, and the rest were mostly in high school (but all over 18). How they managed to miss it for so long is a mystery, considering the title was Loving Families and Naughty Schooldays, which would seem to provide adequate clues about the contents. And, if the title didn’t, the blurb certainly did, in the usual, carefully implied but not quite stated way. You can still buy the book, but you have to get it from Lot’s Cave now.
I love the name, Lot’s Cave. I went to one of those ridiculous Christian schools back home in Atlanta. You know the ones, where the cheerleaders dress like it’s still the Truman administration, and biological diversity is staunchly asserted to be the result of Noah dropping off different animals in different places. We spent a lot of time reading the Bible. Parts of it, anyway. It’s surprising how much is essentially just ancient porn. The story of Lot and his two daughters is one of the better incest stories.

In case you don’t remember, after the destruction of Sodom and Gomorrah, Lot and his two daughters took refuge in a cave near Zoar. Hiding there, they believed the entire world had been destroyed, not just those cities, and there’d be no one for them to marry. Since the two daughters, who were presumably the same two virgins their father had offered to the rape gang outside his house a few days earlier, wanted children, they decided to get him drunk and sleep with him.
I really don’t believe this for a minute. Leave aside that I think most of the Bible is just a collection of old myths, and has about as much to do with reality as Dracula, the situation just doesn’t work for me. Have you ever tried to have sex with a guy who’s so drunk he’s passed out? He needs to be reasonably sober, and conscious, if you expect him to get hard. So, just based on that, I’d say Lot knew what was going on. Given the anti-feminist stance of the Bible, it wouldn’t surprise me if the whole thing wasn’t his idea.
That’s the thing. If you use the Bible as a literary guide, you can reasonably include all sorts of perversions. Abraham was married to his half-sister, Lot fucked both of his daughters, all four of Jacob’s wives were sisters, two full, two half, Song of Songs spends a good deal of time rhapsodizing about breasts, love, and, if you’re paying attention, cunt licking. Reading between the lines, it seems fairly obvious that Jesus was gay (never married, spent all his time hanging around with a dozen guys, and just before his death turned over responsibility for caring for his mother to the disciple he “loved”). You’ll never convince an evangelical of it, but it’s really kind of obvious he was more into guys than gals.
I don’t know if I’ve actually answered the original question. I tend to go pretty far in my writing, mostly involving incest and pee play. Others may not. It really does boil down to a question of where you want to sell. One Room, which you can find on Amazon, is about two completely unrelated lovers. Freaky Farmers, which is only available from Lot’s Cave, triplets having sex, a guy screwing his mother, a hot babe sucking off her brother by the pool, and two sets of siblings (and cousins) screwing around both separately and together.