Why does Vivien say that she "shan't tell" Lancelot's identity to her friend in King Arthur's Socks: A Comedy in One Act?

I think this is the part when Vivien was talking about wanting to get married. However, as she was talking to her eager friend who wants to know as to whom she wanted to get married, she told her 'shan't tell' because she doesn't want to appear like she has 'the nerve to want him'. In other words, she didn't want to look desperate about it.