Can Married Couples Commit Fornication?



There is a commonly suggested theological summation with reference to 'fornication' as it pertains to the 'putting away/divorce' of one's wife that a 'married couple' CANNOT commit fornication.  As such, many use the following argument as the basis for their theology. 

          "Joseph was about to put Mary aside for fornication, even though their marriage had not been consummated, but he was allowed to do
            that without any liability under the law,  but had their marriage been totally consummated, he could not have put her aside for any reason.
            because a married person cannot commit fornication in the marriage sense of the word.  While there are many forms of fornication, if a
            man or woman who is married has sex with another person, he or she commits adultery, NOT fornication."  

With respect to the above quotation I humbly ask that those who support this theory PLEASE answer the following questions: 

Do they think that ANY of the individuals in the following Passage were ever 'officially' married (per their definition of marriage - marriage sexually consummated) before they committed their wicked acts of fornication OR do they suggest that NONE of these peoples had ever been officially married? 

          Jude 1:7
          Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange
          flesh, are set forth for an example, suffering the vengeance of eternal fire.  

We know from Scripture that the above noted sin of 'fornication' was indeed both physical and sexual. 

Meanwhile, how do they explain these Passages referenced by Apostle Paul and do they think that ALL 23,000/24,000 Jews who committed this act of fornication were actually ever 'officially married' per their definition?  They were committing Hebrew 'zanah' - fornication/whoredom/idolatry BOTH physically and spiritually with the daughters of Moab and their pagan gods. (Num. 25:1-9)  

         1 Cor. 10:8 (emphasis added)
         Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.  

         Numbers 25:1,9
         And Israel abode in Shittim, and the people began to commit whoredom with the daughters of Moab... And those that died in the plague
         were twenty and four thousand.  

In closing, are they also equally suggesting that the 'commonly reported' practice of fornication with respect to 1 Cor. 5:1 (having one's father's wife) reasonably assumes that there was absolutely no chance that the father could have still been living when this egregious act of fornication was committed against him by his son?  

       1 Cor. 5:1 (emphasis added)
       It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that
       one should have his father's wife.  

In order for the above suggested theology regarding fornication as being NOT applicable to 'married couples' (after the marriage is sexually consummated) to be sustainable, then they are compelled to answer the above questions with Biblical proof and clarity supporting their preponderance.