IMHO, незамужная is the unmarried woman who wishes to get married, холостячка is a maverick who can’t care less about marriage; it’s a defiant attitude to what is usually expected from a woman. Another usefull word cтарая дева is a spinster of about forty or older who still (c)overtly hopes to marry sb.
PS What a fashionable fancy bonnet, made out of cock’s plumage, btw!