>>1393Writerfag here, has had and gotten paid from having his smut actually published. It's hella late here so I'll give you a quick rundown.
EVERYTHING IS GAME. And I mean fucking EVERYTHING. As expected the actually horrid stuff (toilet, snuff, ageplay, bestiality) are out of the question but anything else that your dirty fucking mind can come up with can be published everywhere.
Generally, a publisher will specify what kind of stories they want according to their Heat Rating (goes from 1-5 with 1 being vanilla sex to 5 being a family BBQ orgy at uncle Dave's). Some have specific limits, but you would be better off browsing the stories that are sold by authors according to heat ratings on Amazon. You'll find some great smut there.
As for dinosaur erotica and 50 shades, that's kid's stuff that got famous for either being parodies (in the first case) or pandering to closet-slut housewives (second). Don't confuse their appeal with the actual content.
Hope that helps,
Anon