It depends on the people writing it. Generally things like that are sort of sorted out ahead of time in terms of what people are willing to write. Some prefer to fade to black, others might write out the scenes in a pg-13 sort of way, and some will write out explicit scenes. That said, the writing itself could be like sexting, or it could be written out more like a scene in a novel. It could be back and forth, or it could be written collaboratively without the back and forth. It might not even happen in game; some people choose to use another method (a chat program, email, whatever) for such scenes.
