True, there would many very interesting opportunities if NTR (or just other guys having sex with free girls) was included into the game to some extent. But some of the patrons are just too greedy.
I mean what's the problem with, for example, Victor fucking Katelyn, if you decided to follow Claudia's or Riley's routes where there is no content with Katelyn and the player himself? Or having a bad end where one of your women is stolen from you if you make some terrible blunder (just don't make it next time, but it certainly adds some excitement and interest to the game) ?
More than that, If those things were optional, there would no harm whatsoever - you don't like them, you don't play them. That's all.