That's a silly question!
If you had to be married the first time before having sex what makes the second, third, fourth time any different? You are supposed to be married in order to do married things.
Sex after marriage just isn't supposed to be wait to have sex after married just because....its don't have sex with anyone unless you are married to them, whether or not you've been married before is besides the point.
If a husband divorces his wife for any cause other than adultery then he is guilty of adultery if he ever has sex with anyone else... ever!
If your ex-wife then marries another or has sex with another you will also be judged for her adultery because you put her away.
If you have sex with her after your divorce but neither of you have had sex with anyone else then I would think that falls under the category of fornication.