My oldest (she just turned 18) just "knew" from way things flowed.
The middle one lives out of state with his grandparents due to some psychological and behavioral issues he has that endanger other children. He's unaware.
Our third child is 9 and I had a specific conversation with him becuase he was struggling with communication and needed some guidance. he felt that what he needed to say was "too hard" and so we talked about how important honest communication is and it led through the relationship dynamics in the family.
Our youngest is only 2. She's well aware that I go to be with daddy at night andthat sometimes in the morning she wakes up and I'm in GreenGecko's room, not Daddy's and mine (GreenGecko is my BF). She is young enough it just "IS" what it is.
I think it helps if your LIFE is honest and open. If you keep secrets about your lifestyle from most of the people in your life-it's going to be complicated with the kids. If you are open about it-then they just see it as "the way it is".
Mine haven't had issues with other people saying/doing anything. But it's not a huge topic of conversation. They don't wander around talking about it and we don't walk around making out in public-so people don't usually ask...
"Love As Thou Wilt"