I like the attitude taken by many swingers that we can be open and encouraging that sex is a wonderful thing, and assure our children that we have healthy and happy sexual relationships, but we do not need to provide details, as we wouldn't if we were monogamous either. Kids don't want to know the details of our sex life. They want to know what sex is and if we are happy, I think. Or as a friend says to her children, “If it is important to you to know that information, I will explain it to you. But first you need to ask yourself if it is really something that you want to know.” We have to be careful in providing more details than kids are ready for.
I do worry as my children are entering their tween and teenage years what they will notice about our friends, our entertaining habits, etc. But I think the rules from above still apply – I think most of all they want to know that we are happy and solid in our relationship. And I will stress that love and happiness occur in many different structures and there is not one right way. Their interest in sex and what it is tends to be is more about them than about us, so I'll talk about sex on their terms, not mine. I think we can be sex positive but private about our own relations – and doing so is not contradictory.