Is it okay to have an extramarital affair?

No, it is not okay to have an extramarital affair. The person committing this act is betraying the trust of his or her partner. I understand that feelings and emotions change over time. However, you should still stand by your obligations and hold responsibility to your partner. In the worst case scenario whereby there is really no more feelings for the other half, then I believe it is only fair for them to get a divorce before getting together with another person.