"THOU, SHALT, NOT COMMIT -"
Moses and the Ten Commandments by Tissot
Public DomainMoses and the Ten Commandments by Tissot
The signwriter is in the process of writing up the seventh commandment: "Thou shalt not commit adultery". This appears, along with nine other commandments, in the Book of Exodus in the Bible (20:14). The Ten Commandments were given to Moses by God as rules for the Israelites to live by. 

The word adultery in this context has a broader scope than we usually give it, and encompasses any form of sexual intercourse outside marriage. No doubt this is what rattles Tess.