When you lie to your partner about where you've been and who you've been with, that is "cheating" because it's being dishonest and unilaterally disregarding the relationship paradigm. Fortunately, there are ways to engage with other people openly and honestly so no-one needs to do that. Polyamory does not condone or justify doing things behind your partner's back (that's still cheating) but instead creates a relationship paradigm where you don't have to. Then you can have all the walks on the wild side you want with no issue.