People like to place blame. It’s one of the ways we try to understand the world and how bad things happen. Because of this, it’s extremely common for the wrong person, event, or thing to take the blame, especially when the real blame could fall on your own actions or the actions of someone you love.
When you have a relationship outside of the cultural norm, some people will blame the relationship, not the actual problem, when issues arise. If a monogamous couple divorces or breaks up, people assume there’s some sort of problem that causes this, such as different opinions on having children, or religion, fighting over many things, or even cheating. No one assumes that monogamy is the issue, even in the case of cheating.