I also think if you believe in anti-monogamy that you cant believe that cheating is a real thing because it would also be a social construct if monogamy is a social construct.

Cheating is being dishonest with your partner. Social construct or not, most can agree it's wrong to be dishonest to a romantic partner. Even monogamous people who have been cheated on often report that the sex itself isn't what bothered them, it was the lies and deceit.

So when you're anti-monogomy and anti-cheating you're really anti-monogomy and anti-dishonesty. Which I would argue is the feelings of most polyamorous people. In fact, extreme honesty is very important for it to work! Furthermore, polyamorous people make different agreements and can still cheat by breaking those agreements. For instance, hypothetically a couple could agree to an open relationship with the requirement that condoms always be used with other partners. If one partner then has unprotected sex they have cheated, because they broke the terms of their agreement.