What metaphors are used in The Critic by Richard Brinsley Sheridan?

When Dangle complains of how comedies have been purged of all "double entendre" and "smart innuendo," Sneer responds with a metaphor that reflects his judgmental mind and style of speech:

Our prudery in this respect is just on a par with the artificial bashfulness of a courtezan, who encreases the blush upon her cheek in an exact proportion to the diminution of her modesty.