Inevitably she will cheat on him, if you allow him to cheat on her. There are obviously going to be consequences for that eventually, with netorase likely morphing into netorare. In the overview, it does suggest their relationship could become a nightmare for Luke, if he makes too many bad choices. Which probably means he will end up getting cucked.
There's also the sex drugs to take into account. At some point, Kathryn could take or be given one of these, and the consequent loss of inhibition could cause her to cheat on Luke. When Rika took one during this update, it turned her into a crazed nympho, almost instantaneously. This might be a pathway to what you suggest above?