Is it okay to have an extramarital affair?

I think that if you plan on doing it then why would you get married on the first place? If you want to be in an open relationship then you can just live together or get married but not under a religion, just do a civil marriage. The moment you get married under the umbrella of a religion then it means that you will live under it's rules, so if it says that extramariatal affairs are considered a sin, then they are a sin, is pretty simple.