Is it rude to ask if someone's child lives with their SO?

An acquaintance of mine at my gym just told us girls that her son got engaged last weekend. I asked if she likes his girlfriend, and she does, they have been dating for 2 years. Someone asked if they will have a big wedding, and she was not sure. And, then I asked if they already live together, and she rolled her eyes and said yes, as in she does not approve or is embarrassed to say so, and the other woman on the conversation said to me I shouldn’t ask that. It seems stupid to me. They do live together. They are a wonderful couple from everything I have heard. College educated, starting out their lives as adults ina productive way. I don’t get it? I would get it if we lived in some small secluded religious town where this would be unheard of, but come on.

So, is it rude? I only asked wondering how much of an adjustment the whole thing would be for everyone. She was giving us details and cute stories about them already. It wasn’t like she was all tight lipped about their relationship.