I have to agree with SJ here. If you break down and go with the intended meaning of affair, poly is all about affairs if you are married or in an LTR.
Affair=having relations with someone who you are not married to.
Cheating=lying or misleading to get what you want.
What you have described, not telling the primary partner that you are having relations with someone, anyone, is cheating. The only time this is not cheating is if you have an agreement with the SO that all relationships are Don't Ask Don't Tell. That I believe is usually uncommon in poly though more likely in a sexually open relationship that is not for the purpose of emotions as poly is.
But also as SJ says, this is just my $0.02.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.