I think you can use your imagination. But in something related, many many years ago, there were two men who were chavrusas, who learned at home, and the wife was always around and in the end she divorced her husband, the chavrusa divorced his wife and they married each other. If your imagination doesn’t suggest anything, would you want your wife to get too familiar with your friend?
I am sure this does happen , and probably happend more than once, but how about the following could also happen. Shabbos can be long and while married people get along, sometimes cabin fever sets in and they start to get mad and fighting with each other (Especially on a long winter night or a long summer day). Having other people over can calm this tension.