Yes, opening the door while wearing only boxer briefs is generally considered inappropriate in most social situations. When you answer the door, you should be dressed appropriately to receive visitors, even if it's unexpected. Wearing only underwear might make your guests or anyone else at the door feel uncomfortable or awkward.
It's essential to maintain a level of modesty and considerate behavior when interacting with others, whether they are expected guests, delivery personnel, neighbors, or anyone else who might come to your door. Putting on a robe, a shirt, or any other covering before opening the door is a simple way to ensure that you present yourself in a respectful and appropriate manner.
Remember that being considerate of others' feelings and comfort is important in maintaining positive social interactions and creating a welcoming environment for everyone involved.