I must admit that I've done this myself to people who were in long-term relationships, just because it's so common in society today, and I guess I also didn't want people to think I was being judgemental. So I think I've had this tendency to assume that people who were going out for a while must be sexually active and that they very well could be or were planning on cohabiting.
I can think of two examples right now in my life where I wound up with egg on my face and I'm sure there have been more:
Example One: I met a young lady (who I think was Protestant, actually) who was thinking of moving to the city I lived in. She mentioned that her boyfriend was already living there. So I asked her, "Oh, so you'll be staying with him?" Well, the lady seemed quite offended and told me that she was staying at a hotel for this visit and wouldn't move in with her boyfriend unless they got married.
Example Two: I met a young man (definitely Catholic) who mentioned going to check out condos with his almost-fiancee (no ring, but "we know we're going to get married"). I don't recall exactly what I said but I did say something assuming they'd be moving in together, and later in the discussion he said the plan was for him to move in first, and "then she can live there...AFTER we get married!" While he didn't seem offended, it did seem important to him that I knew he wasn't planning on "living in sin".
So after these experiences, I've decided it's not a good idea to assume people are living together. I do feel very bad for causing offense before. It's also occured to me that in the future I might be faced with the same issue (people thinking I'm cohabiting with a boyfriend when I'm not). So, I was wondering how people here have dealt with this assumption.