Actually I've never been entirely clear why Lana Lang didn't leave Smallville with him. I know the Byrne explanation (he had no actual romantic feelings for her, which is sort of bizarre, but at least makes sense). But this is one of the more constantly rebooted things about Superman, so I don't even know if they were together or not anymore.
The point is, the only two options that work for me is for there never to be a romantic relationship (which sort of undercuts the major value of the Lang character, and yeah I know that's about as misogynist a thing as I'm ever likely to say, but otherwise she's just Pete Ross with more name recognition and a bit less whitebread blandness, and additionally you have to ask the question "why not?"; I like the surrogate mommy panel transition Byrne did, though), or for Lang to kick him to the curb, which makes a lot more sense (i.e., "we can never have children. Never have more than this," and also "I plan on being a terrible, terrible mate given that I will spend large stretches of time in outer space, dying of kryptonite poisoning and Doomsday fists, and wandering around America being smug.")
But afaik they've never taken the latter tack, though I might be misremembering or merely be ignorant of more recent developments.
Why can't it just be a normal teenage romance that ends as you grow older and grow apart? Clark was never Superman to Lana; she had no desire to be a part of who he was destined to be in any romantic sense.
I dunno, in the Byrne continuity she was smitten with Clark, and the modern take on Superman has emphasized the identity of of Clark Kent and Superman.
Of course, going further back, she was smitten with Superboy, although perhaps less smitten than she was crazily obsessed with his secrecy and lies, before she became more recognizably smitten with Superman and expressed her love in the arguably even less healthy manner of punching Lois Lane in the face whenever the opportunity arose. But we can probably disregard pre-Crisis stuff, where everyone was an utter lunatic.
To answer your question, yeah, if you take two regular folks graduating high school, then even in the best case they're likely as not to grow apart as life takes them in different directions and distance and ambitions separate them. The thing is, this doesn't really apply to Clark Kent much, does it? Distance is essentially meaningless, and the ambitions Clark has aren't really incompatible with anything Lana might choose to do, unless you have her say, "No, this fundamentally sucks, I can't be happy like this. I need someone less important to the world than Superman, like the President."
And a worse case would undermine the nature of the characters. The human foibles that doom most romantic relationships would be difficult to apply. They can't really be written as fundamentally small, or petty, or shallow, or jerks, or stupid, or mean, or unfaithful, or annoying, or you lose something important.
P.S.: Poor Pete Ross. Then again, most of us have been, are, or will be second choices, if we really cared to scrutinize it. And placing just after a Kryptonian, hey, that's a Silver Medal worth bragging about.
P.P.S.: This has nothing to do with anything, but have you guys ever read the one where Superman explains to Supergirl, almost lamentably, that Kryptonian law forbids cousin marriage? It's hilarious. Aside from the obvious, it's also funny because 1)he's from a 1930s farming community, which are well-known for their blanket prohibition on consanguinous marriage and 2)he's the default executive, legislative, and judicial authority of Kryptonian civilization anyway, so it's a pretty stupid thing to appeal to.