"Being poly" has nothing to do with sex, or how many sex partners you have.
"Ployamorous" mean that you are capable of feeling romantic love for more than one person at a time. You are either capable of this, or you're not. Ultimately, sex is a part of expressing love between adults, so the two are intertwined, but not mutually exclusive.
Being in a polyamorous relationship means that at least one person is allowed to express their polyamourous nature, either by having a fully committed relationship with other people, or by having an intimate and close friends with benefits relationship with other people.
Not everyone within a poly relationship is poly themselves, nor does everyone within a poly relationship date/sleep with multiple people, nor do they all date/sleep with each other.
Yes, there are poly relationship groups out there do share communal sex and love, but this is not an accurate example of all.
Having sex outside of your relationship is not "being poly". Unless you're feeling love for both people, romantic love, not limerence or infatuation from the sex, then you're not poly.
Polyamory is not a valid excuse for cheating. Cheaters are acting selfishly, without love, when they cheat. But this does not mean that polyamorous people aren't capable, only that they should take responsibility for their choices.
Polyamory is not an excuse to "play the field", or avoid commitment. Truly polyamourous individuals are committed in some way to each and every one of their partners, as they feel genuine romantic love for all of them.
Non-Monogamous individuals do not attach through sex, polyamorous individuals attach before sex, and usually form strong bonds before sex, the same as monogamous couples. To a non-monogamist, sex is just a physical act. It's not romantic, or spiritual, it just feels good, and maybe they want to try out someone new once in a while. There are no emotions involved, whatsoever.
Non-Monogamists do form romantic attachments, just not through sex. They fall in love through bonding behaviors and displays of affection, just like monogamous couples.
Polyamory and Non-Monogamy are not a choice, they are part of who you are. For many reasons, someone might simply choose to remain single and have multiple sex partners or an endless pool of dates to call up and take out as arm candy.
You can be completely monogamous, and still be in an open relationship.
If you are monogamous by your own choice, allowing your partner to go outside of the relationship doesn't change you, as you are not sleeping with, or dating other people, yourself.