The main focus of the game is Kate anyways, the Lesley and Johnny thing just serves a plot device/foil to further there story. Still hoping that we get Johnny as the next antagonist if ever this one gets a sequel because HOOH boy, do I want to see the comeback of an underdog.
Honestly I blame both Johnny and Kate for letting this sort of thing happened, we already know that Kate is doing this because he doesn't want the only family member of hers to leave her behind in WHICH will be fulfilled by Dexter in the upcoming patches. As for Johnny if he just decided of his own to be in the military and actually man up for both Kate and Leslie then he could still probably redeem himself (granted Johnny has to be literally soulless and spineless asf because it wouldn't be a hardcore NTR game if the MC actually decides to fight back).
As for Leslie, I really couldn't careless if she's with Gavin because fuck that bitch

.