I have no desire to keep my children out of the loop. I have a 7 year old and a 5 year old and they are both very aware that their mom and dad have others in their lives who they love. We've never "come out" to them as such with any kind of formal talk we just live our lives. For me, anyway, I think that trying to keep things from your kids is probably far more detrimental to teaching them about relationships. If you are hiding things from them you are teaching them that you think that what you are doing is shameful and that there is something wrong with letting yourself love freely. As long as you are treating your partners with dignity and respect your child will learn to love the same way.
Do you have a dream that your child will grow up and marry monogamously and live happily ever after? Who knows, your child might grow up to be heavily involved in a lot of outside activities and might not want a full time relationship and may be perfectly happy being someone's something on the side. And there's nothing wrong with that. Live your life with integrity, answer your child's questions with honesty and everything will work it's self out.
Everything will be ok in the end. If it's not ok it's not the end.