What is the significance of the astrological sing of Stanley and Blanche ? Scene Feive

Blanche is trying to make conversation. Blanche finds out that Stanley is a Goat (Capricorn). Stanley asks Blanche what her sign is. Blanche says it is Virgo, the Virgin. At this Stanley scoffs conveying the irony of her statement: he is relating to Blanche's promiscuous past.