There's no legal reason why a person can't become engaged before they've completed a divorce. (Indeed, since 1970, when a woman's right to sue for 'breach of promise' was removed, the concept of engagement now has almost no legal meaning anyway. The law only deals with the return of gifts if an engagement is broken off).
I thought that Maggiebee might welcome the old-fashioned idea that if a man gets a woman pregnant he's morally obliged to ask for her hand in marriage.