With our teens-we introduced them to the term polyamory and it's definition.
Then we explained that it was an important idea to us and we wanted them to know that it's ok to love more than one person.
Then we explained that if they saw whichever one of us kissing/hugging/loving on someone else that we didn't want them to think that there was a problem or a secret etc.
We let them know it was totally ok to ask questions-if they had them. But that we mostly didn't want them to feel like "oh shit I just saw mommy kissing santa clause! Do I tell dad or not?"
They all took it well (the older ones are now 21, 17, 16 and it's been 4 years).
"Love As Thou Wilt"