Is it okay to have an extramarital affair?

It would depend on your situation. If you truly love your spouse, I don't think there would be a motivation to do that especially if you are contented with your marital relationship. Most of the people I know who have extra marital affairs are men who are not contented with their wives. Cheating on the spouse can have a drawback someday particularly when you grow old and you feel guilty for what you had done before.