Spider-Man AKA Peter Parker
Mary Jane Watson
Spider-Man has a certain Hugh Grant-esque bumbling sweetness about him, and legs that go on for miles.
Spider-Man's a wisecracker. Everyone knows that, it's one of his defining character traits. However, audiences only have to tolerate Spider-Man's lip for 28 pages every couple of weeks, or a couple hours every few years. Imagine trying to live with the guy. Imagine trying to have sex with the guy:
"So I guess that's where I left that web-shooter!"
On top of that, he's a terrible provider. Costumed vigilantism gets you tons of headlines and the adoration of millions but the pay amounts to all the spare change he can salvage from window ledges and pigeons' nests. The good news? That's still more than your average freelance photographer pulls in.
Spider-Man sold his marriage to Satan. Just read that sentence over a couple more times, let it marinate. Done? OK, allow us to explain:
Spider-Man, in one of his trademark haphazard attempts at doing the right thing, unmasked himself on national television. This led to the Kingpin putting a hit on the wall-crawler, and Aunt May eating the sniper round intended for her dimwitted nephew. With his former caregiver in a deteriorating coma, Spider-Man's angst began approaching critical levels, attracting the attention of the demon Mephisto.
Mephisto, feeling generous, proposed a deal to Spidey: Aunt May would be returned to full health, bullet-free, but in return ...
... he would have to give up his marriage to Mary Jane. For some reason.
Spidey hemmed and hawed for a while, but eventually came to the logical decision: Abandon countless future years of happiness with the woman he loves in exchange for giving his octogenarian aunt a chance to die of heart failure or something.
The marriage of Peter Parker and Mary Jane Watson could be likened to a Toyota Prius with faulty brakes: Comfortable and reliable under most circumstances, but destined to eventually flatten itself against a brick wall. And ultimately, not worth it.
The Incredible Hulk, AKA Bruce Banner
The alien warrior queen Caiera.
Say what you like about the Hulk, but he's reliable (to be fucking insane with rage.) Great teeth, too.
Aside from the obvious (that he may flip out, grow six feet, and demolish an entire city block because it was suggested they're getting a little paunchy) there's anatomical compatibility to think of. Bruce Banner may have the tiniest, most adorable member imaginable, but once he Hulks out, that thing becomes weaponized.
"Weaponized" is actually a literal description. Every single fluid the Hulk produces is so radioactive that a Hazmat team would rush into the bedroom every time he climaxes.
The Hulk is cursed. A happy Hulk is not a Hulk at all, so in the comic book universe that means bye-bye to anything that might bring him peace (like, say, a happy relationship.) This is bad news for anyone hoping to settle down, maybe buy a house in the suburbs, and pump out little Hulks.