As is the case here. It's difficult to see why anyone would amicably break up with either of these two. It has happened in the past, with Trevor Barnes and Diana, but the chief factor was there seemed to be he didnt feel super enough to handle her adventuring lifestyle. That can hardly be a factor here. People seem to be spending a lot of energy talking about how perfect these two characters are for each other. So again, why would they break up? Because they develop feelings for someone else? Unlikely. Because they find themsevles no longer attracted to each other? Again, seems a remote possibility.
The most likely scenarios seem - universe changes [again] or they come to some sort of crisis, possibly fueled by Diana's current willingness to slit people's throats.
The other problem, which I think I may have mentioned before, is the need then for Diana to date Batman or become a dedicated lesbian. Because frankly no other guy is going to be pscyhologically prepared to fill those big red boots in the bedroom/mountaintop/cloudbank/antarctic/volcano or wherever else writers are likely to show these two doing it. What it would be like for a woman in that situation with Clark I cannot imagine.
No doubt folks will say not all men are as insecure as me. But we are not talking about measuring up against Daniel Craig or Chris Evans here. That would be difficult enough, but how do you compete with a man who can give a woman an orgasm that literally causes earthquakes? Sorry, that is a bit much to ask of someone even as awesome as Steve.