Actually Natasha called herself a monster because she was trained to be a killer. The sterilisation thing was a separate issue. She was sympathising with Banner after he said he can't have kids so she probably wouldn't be happy with him.
That is terribly optimistic of you. I very much got the impression that she was referring to her sterilization when she called herself a monster. When she was talking to Bruce she said in her graduation ceremony she was sterilized which was effective in making sure nothing was more important than her missions. Then she called herself a monster. The way the scene was written made her sterilization and her being an effective killer too wrapped up together to say she was only talking about one thing.