My 16 year old son is oblivious and my 17 year old daughter already suspects. Of course we've always been fairly open about sexuality around our kids in terms of answering their questions and letting them engage us in a dialogue (regardless of how uncomfortable it might have been). I was shocked when I saw my daughter had put on her facebook page, last year, that she was in an "open relationship", because her bf lived 500 miles away. Hell, dad didn't even know she had a bf. And when I asked her to puh-lease remove it from her facebook page she asked me if I'd ever heard of polyfidelity. I just said, "maybe, why don't you try to explain it to me and I'll let you know if it's what I'm thinking it is." Wow...she hit the nail on the head. Dad just sort of bit his tongue, nodded, and said, "hey, whatever floats people's boats and whatever they're comfortable and happy with themselves...that's what matters." I then went back to hide in the bedroom and wondered if maybe the wife and I had been just a hair too open in our conversations. I don't know that I'd start an innocuous conversation out of the blue. They were raised on Law & Order: SVU, watched "The People vs. Larry Flint" while fairly young, and we do not censor their internet surfing at all. We've always let them start the conversations, and we just probe a bit with some open ended questions and (hopefully) insightful comments. I think everyone is going to have to just sort of feel this one out themselves and look at their childrens' personalities, openness, and just see what works for them. BTW, both of ours know we've *experimented* with different things in our past, but do not know that we've started *experimenting* again.
Best of luck!