Um…I don’t know that rather random sefer, but can we close this discussion about mixed swimming with one’s spouse? It’s mefurash in Halacha that it is obviously permissible; I am loath to quote the Rambam & Shulchan Aruch about this. Please, remember, something is permissible until proven otherwise. There is a halachic framework; it doesn’t work based on random seforim. There is a process.
Moshe’le, please leave the hanahgos of couples to the privacy it deserves. There is a whole realm of Halacha here which needs to be left to those who are married, in private halachic discussion. I presume it’s not relevant to you.
In fact, I appeal to the MODS on this. We tread dangerous ground.