The writer's use of the word 'cheat' and 'infidelity' are appropriate to the prevailing social mores.
I mean, the article it links to expressly
uses the terms, so it would be political on his part to change it.
He also talks about slut shaming without actually casting judgment! IE, not slut shaming in the article.
There is however a valid point to the use of 'cheat' and 'infidelity', in both articles; in the human culture these people have (normally, supposedly) made a commitment to not
engage in extramarital activities, so in the sense that said commitment is broken, they have cheated.
We are supposed to abide by the terms of this forum, for example, and if we don't it would be fair to require moderation, banning, or reprimands.
If you don't want to see 'cheat' and 'infidelity', you need to be part of a movement where marriages aren't used to promote monogamy.
Even then, fidelity isn't actually a bad thing. It's protective against STDs, careful allocation of time/resources, and stability. I imagine if there were no commitments then there would be less incentive to invest into a relationship, or kids. It's not unlike how a car loan is a 5 year commitment to paying off the car; a marriage can be thought of like a multiple year commitment to raising kids and forming a family.
The difference of course in a poly sense is that there is the idea that you can have multiple simultaneous commitments, no different than owning (so long as you pay!) 4 cars at once.