Here's what I remember... I read it when I was young, so nothing published after the mid-80's A couple purchases an old door that turns into a passage into another world (standard fantasy stuff). There was enough sex for me to remember that aspect of the book, so not a YA title. On the other side of the door the society engages in a mild kind of slavery, mild because people choose whether they want to be master or slaves. No one is forced, which... sounds kinda D/s and might well have been but I was too young to twig. Masters are noticeable through their wearing of gold jewelry items. There are animals they use as riding beasts that look a bit like a cross between a polar bear and a chalicotherium. I could swear the book was called The Tesseract or just Tesseract, but there's another book by Alex Garland with that title that is absolutely not the book of which I speak. There are also seem to be numerous other similar or same titles that have sprung up since the 80's... Sound familiar to anyone?