Does not compile. They will not stop dating their partners just because you are pursuing them.
Helen and Dylan stopped doing anything for year. Katie & Zoey are both lewdable girls for the MC, it's just another fantasy porn trope, Gina's boyfriend was a cuckhold in the old version, she even offered William to fuck with her. Is Monica even a love interest ? It doesnt seem so, Willy treated her like garbage.
So no it's not like the girls you mention are actually cucking you because they have a partner. Ultimately, you will be the one who's gonna cuck their current partner ( or more in Katie and Zoey's case ).
BUT just like Miru, some girls might potentially fuck others men in the current time. In that case, it's NTR.