Its all in the Mosaic law. If a married man had sex with an unmarried woman, the worst that would happen if caught would be that he would have to marry her too. Today if a married man has sex with a unmarried woman it is viewed as adultery. This was not the case in the bible, and is not scriptural. I believe that the sexual laws of the OT were more about property rights than what christians today view as morality. The ONLY way a man could commit adultery was if he had sex with a married woman because he would be infrignging on another mans valuable property. If you had sex with a virgin and got caught, you would have to pay the father because again, you infringed on his property. Adultery only worked one way in the OT, if a married woman went outside the arrangement. A man could have sex with as many wives, concubines, and slaves as he could afford. Marriage was viewed much differently then than now, and even today what marriage is varies from place to place. To add another wrinkle into the conversation, if a man had sex with an engaged free woman, they were both punished with death, however if a man had sex with a engaged slave, he would merely have to make a sacrifice,because she was not free. Again if sexual morals were the issue what difference would it make if you were free of slave?
The idea of porneia was not introduced until the NT. Even Jesus did not mention a wife being able to divorce her husband, only a husband his wife. Keep in mind I do not agree with this thinking, I am just seeing the bible for what it is, not what people want it to be. Even the NT does NOT prohibit taking more than one wife. Polygamy is now considered adulterous by JWs and most Bible believers, however that is NOT what the bible teaches. Being husband of one wife was listed as a SPECIAL requirement for appointed men. If this was a requirement for all, it would not need repeating that it is a special requirement. It would be like today the qualifications to be a Elder being that you cannot smoke or gamble. Since no one can, it would be redundant to list it again.
It is distasteful to modern sensibilities, but in the Bible, a man was not held to the same moral standard as a woman the way they are today in the western world. Many try to gloss this over but no one really knows what the word "porneia" really meant. The WT has went from gay sex and bestiality not being porneia, to now where everything is included down to lap dances. They change the definition to say what they want it to say. This is the problem with trying to superimpose bronze age morality on Victorian puritanical morality, or in other cases modern morality.