I think that sometimes we as erotic authors take the easy way out, throwing together a quick sex scene and barely adding story. Its certainly the case on some websites of erotic fiction where you weed through hundreds of poorly crafted stories to find a semi-decent one.
What is fun about playing with settings is they can be as visually impacting, or as glossed over as you want them to be, sometimes the fact that a man and woman is in a barn is enough, sometimes its not.
Making a world believable is not difficult if it is believable to your character. Purple grass if it is real to your character is acceptable, just as fantastic creatures, elves, gnomes, centaurs, vampires, etc. are. Remembering setting is an element that shouldn't be forgotten, just like creating realistic sex, in most pieces, shouldn't be the entire story.
The good news is there is a lot of world out there for your inspiration.
Simply Google landscapes, cityscapes, and oceanscapes. Take five minutes and describe one picture in visual detail - then set it aside. Do three to five of them, and leave them for a few days - then go back to them, look at one and read over it - what do you picture now? Is the scene as vivid as the photo or do you need to add more to make it have more impact?