Polyamory is the philosophy and process of romantically loving more than one person at a time, in an ethical fashion.
Polyamory is about loving more than one person within the rules of the relationship. If someone is sneaking around, or breaking the rules of the relationship, and tries to excuse it by saying, "Well, polyamory is loving more than one person at a time, and I'm polyamorous!" then that person is being a rotten monkey, because they are using polyamory to excuse cheating.
I have had several polyamourous, or attemped polyamorous relationships. Some of them worked well. Some of them did not. I learned just as much from those that did not work as I did from those that worked.
It might be cheating if...
One can certainly be polyamourous, or capable of loving more than one person in a romantic fashion, and not currently be acting on it. One can be heterosexual and not currently sexually active. Someone can certainly be polyamorous and committed to a monogamous relationship. Depending on the cirucumstances, this may or may not work well.
Capacity does not equal necessity. I am personally capable of eating a large pizza with all the trimmings all by myself in one evening. I am capable of coding HTML raw, without helpful tools. Do I necessarily want or need to? Not all the time.
