From what I've gathered, it seems that Ben is single now in 5 Years Later while in Omniverse he was a couple with Kai Green. Did they break up at some point, and if so, what happened between them and after?
Kuro (and the creators of the show) stated that the timeline shown in OV is a alternate POTENTIAL timeline ("Potential" because nothing in the time continuum of the Ben 10 Universe is set in stone.), meaning she still exists but is off doing her own thing, and is not with Ben.
Every time ben meets his future self, that future is no longer the true future, but one thing that’s been consistent through the show has been Ben and Kai ending up with some form of relationship that leads to Kenny being born, wether or not they stay together or get a divorce or even Kenny being an accident in a one night stand, Kenny is almost always the end result in these futures.