Legal Question in Criminal Law in California

What am I missing? As long as she is over 18 and any sexual act is consensual, it is legal.

Are you wondering about prostitution charges? You didn't mention anything about money.

I agree with Mr. Dane, but I see other possible ways to read your question.

You may be concerned that the women isn't really 18 as she claims. If that's what you're worried about, your best bet may be to find another model.

You might also worry that she will falsely accuse you of harassing and/or assaulting her. If so, I don't know how checking her ID or signing some sort of agreement would protect you. And here again, if you're genuinely worried that this is what will happen, you should probably not get involved with her.

If you're just concerned about documenting that your model was an adult when she posed for you, the steps you describe would make sense. But that has nothing to do with whether she might pursue you sexually.

It's not clear what sort of "agreement" you have in mind. What, exactly, would either of you agree to do or to refrain from doing? Perhaps if I understood your question better I could give you more useful guidance.