First of all, you must be prepared to see yourself dressed up in her clothes. In drag, in costume, in spangly eyeliner and a fedora hat. You have to steel yourself, and accept the following with equanimity:
She is going to write about you.
It takes a strong person to bear this: you'll see your private jokes, your secrets, your childhood, the angle of your penis, the heft of your breasts, your personal griefs, your complaints, your house and your profession ground up and mulched, composted and laid out bare, for anyone to see, in her books. Her books are naked, and she will make you match her. It will not be comfortable. She'll use everything you are--but she's fair, she uses everything she is, too.
Every time you touch her, she will store that touch away, to be accessed later, spooled out, smoothed over, given to characters she hasn't even thought of yet. Every time you fight, she will mentally catalogue your turns of phrase. If that seems inhuman, well, she can be like that. Computers are not so ruthless about retaining information.
Of course you will have to comfort her when she has gotten rejections, tell her she is brilliant at least seventeen times before she believes it again, suffer sympathetic pregnancies every time she struggles to bring a book into the world. You will have to feed her when she forgets to do it herself, and bear up under infrequent money, critical disasters, and her quitting the whole business at least once a year. And naturally, if you fail to love her books, or pretend to like them for the sake of peace, she will never forgive you. Until the next book, which is so wonderful, not at all like that one you hated.
You didn't hate it. But she won't hear you.
If you leave her, cheat her, crush her, neglect her--she'll write about that, too. Even if you think didn't do any of those things. And even if no one else knows who she means when she uses that fateful pronoun, you will know.
If you stay with her, you will eventually see pieces of yourself, tiny slivers--the way you take your coffee, the way you talk to your mother, the way you rub your eyes when you're tired--exalted and debased, deified and debauched, in any number of stories, any number of heroes, any number of villains. You will hardly develop a new habit before she's written into her heroine.
If you stay with her, you can look back, in thirty years, forty, and see how well she will have preserved your lives together, how faithfully, how accurately, and with such variation. Butterflies under glass. In her very secret places--which are only pages, where she keeps all her most secret things so that she can show them to the world--she will have dressed you both up as everyone in the history of the world. You could never have lived such lives as she has written, full of griffins and monarchy and grief.
You cannot castigate her for any of this. She will just smile at you and say: "Why are you so angry? It's only a story."
And there is no arguing with that--it is only a story. But it's also you, your soul and your marrow and your life with her, and she knows it, but she won't let you spoil the tale. You made the choice when you slept with her the first time--you belong to her, and she will use you as she pleases.
My advice is not to do it. What sane person would sign up for that?