Proverbs 6:32,33&34

32 But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul. 33 A wound and dishonour shall he get; and his reproach shall not be wiped away. 34 For jealousy is the rage of a man: therefore he will not spare in the day of vengeance.

Note: The verse is talking about another man's woman. If there were no other man then certainly there would be no man to be jealous. HOWEVER, outside of marriage an act is fornication. Do not fall into fornication. The Bible says "But if they cannot contain, let them marry: for it is better to marry than to burn."_{1 Corinthians 7:9}
   Therefore, do not commit acts outside of your own marriage whether you have one wife or a dozen wives you still do not have the right to perform acts with women other than the women you are married to. That would be fornication.