Ribbi 'Aqiba taught: when husband and wife are worthy, the Shekhinah dwells between them; when they are not worthy fire consumes them (Sotah:17a). The words "Ish" (man) and "Isha" (woman) share two letters, Alef and Shin. The remaining letters are "Yod" from Ish and "Heh" from Isha, which together form one of G-d's names. The numerical value of this name equals 15.
The Keli Yaqar comments that it is interesting to note that the number 15 appears repeatedly in Parashath Nowah, such as in the verse: "The waters arose fifteen cubits above the mountains" (Bereshith:7:20). The number fifteen is also hinted at in the measurements of the Ark, and elsewhere. Why is the number 15, which is the value of G-d's name, specifically mentioned in this Parasha?
The generation of the flood exhibited corrupt and immoral behavior in their relationships between men and women, thereby desecrating the Holy name of G-d, because it can only rest upon husband and wife when their moral conduct is appropriate.
The Hakhamim teach that when the Children of Israel were commanded to keep the laws of forbidden relationships they cried and complained. Even that generation which was on a high spiritual level, living at a time far removed from the 21st century, expressed resentment and grievance. Our physical desires stand in our way and present an intense spiritual challenge in this area of holiness which is extremely difficult to observe.
Perhaps because of this and because of the extreme importance of adhering to the rules of forbidden relationships, each and every single one of us must be careful not to cross the fences that our Hakhamim set for us.