Here is something I always try to remember when interacting with anyone I don't know well about anything: the only information anyone has to go on is what they've been taught, their experiences, and in some cases, their imagination.
So how it relates to your question about the meaning when people don't inquire about your relationship status (aren't aware of ethical non-monogamy, don't care if you're cheating, etc.), it could be any one of those things and some we may not have thought of. Some people may think your status isn't any of their business - that it's your responsibility to take care of you. Some people may assume (naively) that if you return his or her interest, you are single.
In other words no way to generalize.