Because the public sentiment towards open marriages is even more negative than towards infidelity? One is seen as a mistake that one can atone for, while the other is seen as a rejection of the institution itself in a country that has strong beliefs in it. Still only 65% of Americans support gay marriage, for example. Marriage following strict rules remains deeply important to a lot of people.
Absolutely it is performative. Writing a public "apology" for spending time with another person in their free time is also performative. Nobody actually cares what you do with your life. They aren't thinking about you at all. But when performing, the performance has rules of engagement.