I'm not really young (in my 30s), nor inexperienced, and also grew up in a household of all females, so I'd dare say I likely know more about women than most men in general, especially given that I'm still on quite friendly terms with most of my exes.
My entire point with that statement you quoted was that calling KZ bad for possibly seeing other women, yet dismissing KD definitely seeing other men was hypocritical. It doesn't matter if she had viewed the relationship with KZ as serious at some point, if she was seeing men (which, according to the mother, was certainly the case) then she was not being exclusive, and therefor did not consider a relationship with KZ to be serious. Either that or she was pointedly cheating on him (if she considered it to be a romantic relationship), and to say that it's acceptable for her to do so, and demonize KZ for the same behavior (other behavior notwithstanding, i.e. drugs and CP) is very hypocritical. If anything, such behavior should be viewed as possible motive for her disappearance (note I state motive, not, I repeat: NOT, justification).
Edit: by motive I mean to state that KZ found out she was seeing other men, or she told him that she did not intend to have a (possibly further) romantic relationship with him.
Just to clarify, I don't agree with Kelly's choices. Both Kelly and Kris made poor choices. Everyone is imperfect so this isn't that uncommon but they made poor choices to a greater extent than the average person. Regardless of what happened, I feel for both of them. It's a very sad situation.