Kate was raised as a lady. Sienna wasn’t. She was in a trade. She had a profession and makes her own money (through various ways) Benedict could also never marry Madame Delacroix for the same reasons. Also, Kate was assumed to be a virgin and “raised well” and from “a good family” and connected to society. Sienna was part of the demimonde - the society on the fringes of high society (the ton) - a society of courtesans and mistresses of wealthy men.