This is the Doc Savage justification: "I have too many enemies and too many responsibilities to possibly have a wife."
But it's ok to hang out with normal people in his secret identity because his very presence won't endanger them. Obviously, none of his powerful foes could possibly track him down, stomping all over the humans he hangs out with that might get in the way... Phantom Zoners, Darkseid, the Superman Revenge Squad, etc.
Well, who's to say marriage can't work as an extension of "superhero family-building?" Thus bringing in even more of an adventure element. The single greatest disappointment of SUPERMAN RETURNS was, to quote Roger Ebert, "Superman's son should be brassy and fun, like the Spy Kids."
Marriage doesn't necessarily imply children, and certainly not "right away". Think Nick and Nora, or Hart to Hart. Heck, it may not be possible for those two to have children and they may have to <gasp> adopt.
But I think you do have a point: if Superman has time for Clark Kent, he can have time for a marriage too.
More to the point, Superman -makes- time for Clark Kent as surely as he makes time to patrol Metropolis. What's the logical motivation behind that? The traditional motivation, so he could easily get wind of the latest news flashes, doesn't make as much sense for someone who can hear cells divide and who can wield an Internet connection. What's the reason us readers want to see Clark? Is it just to get a supporting cast and a chance to see Clark dodge the "are you Superman" bullet in clever ways, or is there more? Does marriage follow from that?
And by "hoax" I mean deception and closely-guarded secret. I didn't mean to imply Clark wasn't Superman and Superman wasn't Clark, but he sure goes out of his way to meekly and mildly conceal how some of the sides of himself work. To defend the marriage, you have to figure out what makes Superman/Clark tick. That's not so easy, as stuff has changed over time.