Of course it's acceptable.

Here are my views on this. I think that's it's perfectly acceptable for a male to protect a female in a fictional story. But how it's portrayed will influence my view of whether or not it's sexist. If the female is actively participating in her own salvation or in some way attempting to save herself or help the male I wouldn't call the situation sexist. If she's entirely passive and simply waiting to be rescued with no attempt to save herself or to try to think of a way out of her predicament I wouldn't necessarily call that sexist...unless it was a consistent theme throughout the story. It would also depend on the overall tone of the book and how the male and female were portrayed. Is the female consistently portrayed as weak with nothing to offer? Is the male consistently portrayed as strong with no weaknesses? I think the tone and character portrayals play a larger part in determining if something is sexist than the act itself.