I will have to let someone else comment on classification to give a book answer, but I've read a lot inside and outside the genre and I'd say it's more of a quality thing than a quantity thing. There are plenty of Romances that have a fair amount of sex, it's just blushing sex. I know of at least a half dozen science fiction titles with more than four sex scenes. Fantasy? Have you read the Song of Ice and Fire? No one is claiming that Game of Thrones is Erotica. So let me modify my statement above, it's more a quality and ratio thing. GoT has erotic scenes, but they are balanced by troves of swords and intrigue.