Nothing - nor with open relationships!
Some folks see open relationships / polyamory as just glorified cheating. Some just prefer the concept that if someone is in a relationship - especially married - that things should only be between the two of them, relationship-wise and sexually. Maybe they don't understand that open relationships / polyamory requires consent and understanding of all-involved, else it moves into "cheating" territory. Maybe it's a religion thing.
Whatever their reason for disliking non-monogamy, it's irrelevant to the main point, here - that their comment amounts to "I don't like this, therefore it's cringe." It's fine if they don't personally like non-monogamy, but they could have commented differently.