Answer: She believes in adherence to societal expectations.
The Importance of Being Earnest is a play by Oscar Wilde. The play is a satire of Victorian ways, in particular the treatment of institutions such as marriage.
In this excerpt, Jack reveals that he is the lost baby that Miss Prism left at Victoria Station. Joyous at the revelation, he attempts to embrace Miss Prism, at which she replies that she is unmarried. The implication here is that she cannot embrace a man who is not her husband, whoever the man is and whatever the situation they are in. It shows an almost absurd and comical adherence to social expectations.