A married female friend of mine was visiting her hometown on vacation without her husband (she wanted to visit, he was working, it was all good with all involved parties). She was suntanning in a local park when some guy started trying to chat her up. She wasn't interested, but trying to be polite and give cues like inserting "My husband this, my husband that" into the conversation. So he wanted to know where her husband was, and she explained that he had not come with her on this trip. (I know, she shouldn't have felt a need to explain a thing, but that's her nature). So he made comments like, 'Oh, you must have a very open marriage then!" She did not know what he meant by that but it was still offensive to have a stranger making assumptions about her marriage.
Anyway, I think the default assumption should be "not an open marriage" until otherwise indicated.