Why has the "3rd precept" of the 5 precepts been interpreted as being so permissive? It seems to me that according to one interpretation of the "3rd precept" of the 5 precepts one could even engage in various sexual acts with a vast number of prostitutes nonstop and that still wouldn't be considered as breaking the "3rd precept". (I am doubtful of it being possible to attain stream-entry with that indulgent behavior therefore I believe the Buddha did not teach that as being a way to attain stream-entry.) 

Why has the "3rd precept" of the 5 precepts been interpreted as being so permissive?

I find it concerning how the "3rd precept" of the 5 precepts has been interpreted as being so permissive.
I have explained the original Buddhist teachings about sexual conduct &amp; misconduct, before. 

The 3rd precept in the Pali suttas (e.g. AN 10.176) says a daughter is to be "protected" by her family. 


  Abandoning sensual misconduct, he abstains from sensual misconduct. He
  does not get sexually involved with those who are protected by their
  mothers, their fathers, their brothers, their sisters, their
  relatives, or their Dhamma; those with husbands, those who entail
  punishments, or even those crowned with flowers by another man.
  
  AN 10.176


Per the later Commentary, the word "protected" means, per the DN 31 sutta, the daughter is "restrained from unskillful behaviour" until the "parents arrange a suitable marriage" for her into a good family. 


  In five ways, young householder, a child should minister to his
  parents as the East:
  
  (i) Having supported me I shall support them,
  
  (ii) I shall do their duties, 
  
  (iii) I shall keep the family tradition, 
  
  (iv) I shall make > myself worthy of my inheritance, 
  
  (v) furthermore I shall offer alms in honor of my departed relatives.
  
  In five ways, young householder, the parents thus ministered to as the East by >their children, show their compassion:
  
  (i) they restrain them from evil,
  
  (ii) they encourage them to do good,
  
  (iii) they train them for a profession, 
  
  (iv) they arrange a suitable> marriage, 
  
  (v) at the proper time they hand over their inheritance to them.
  
  DN 31


This is no different to the instructions in any other major religion. 

Also, the suttas (Snp 1.6) literally condemn having sex with prostitutes. 


  Unsatisfied with his own wife, with others’ wives he’s seen in tow,
  corrupted too with prostitutes— that’s the way to disaster’s woe.
  
  Snp 1.6


The impression is contemporary Buddhism is so permissive because monks don't want to make any waves with Cultural Marxist Hollywood brainwashed laypeople, which monks view as "metta".