Drinking glasses have a tendency to sweat and can damage furniture by leaving a watermark in the shape of a ring on the furniture.
Always place a coaster or cocktail napkin under your drinking glass before setting it down on a piece of furniture. If a coaster or cocktail napkin is not in sight, ask your host or hostess for one. I’m sure it was an oversight on their part and they’ll appreciate you asking so their furniture is not damaged.
Using a cocktail napkin under your drinking glass also helps absorb moisture so water droplets won’t fall on your clothing when you lift the glass.