The magic of fiction is being able to say "It's only a story," write what you want. Unless it's a first-person narrative about kidnapping Barack Obama step by step and keeping him in your -blam!- dungeon in the basement for weeks on end in which case you'd probably have some questions asked and a visit from the party van likewise with things that are also pretty illegal featuring real people or children or whatever. So actually no don't write what you want, be wary of consequences of what you write. So changes names and stuff. Then you can have your Obama erotic fiction.