The Relationship: Yoshi and Birdo have a relationship entirely predicated on their mutual pairing in Kart Racing, Tennis, and the like.
The Problems: This couple makes no sense. Let's not dwell too long on the gender ambiguity of Birdo (some official pamphlets refer to him as a boy in drag, then Birdo's a girl, then-just kidding- he's a dude again). But we also have to remember the very basic division: Yoshi is a loyal and universally recognized heroic character who protected Mario since birth, and Birdo is that one weird villain who showed up one time. They're not even the same species: they're on totally different ends of the weird dinosaur spectrum. You wouldn't date a Chimpanzee, would you? That's right, not unless she was really hot, which Birdo isn't. Nintendo has no excuse for coupling these two together. After all, they bothered to introduce an entire Yoshi Island so why can't they just magically introduce a love interest from there?
And most worryingly of all, Birdo is fictional well, they're all fictional, but Birdo is fictional even within the Mario universe. Think about it: Birdo first appeared in Mario Bros 2, which took place a dream. If your best friend said "Hey man, I had this really crazy dream which only briefly featured a gender-ambiguous monster reptile which spat deadly eggs at me, and she's coming over later to play Tennis," would your first question be: "Oh wow, are they single?"
The Relationship: They were literally made for each other! Or rather, you made at least one and they just got together or something. But still, that sounds pretty normal.
The Problems: For starters, you're only together in the first place because you arbitrarily found each other and noticed "oh wow, different genitals" which, by the way, neither of you have. Beyond that minor limitation, the guy you made is named "Thugstar" and is an aspiring ninja whose life goal is to have fifty dogs, and the lady is a babbling randomized gorgon who speaks a gibberish language which, based on my experience, doesn't work out.
But I can change, Sims-ex. I can change! Come back to Thugstar. We can work this out, baby.
The Relationship: Duke Nukem, international ladies man pulls in all the ladies with his muscle-arms, guns, and witty bon mots about boobs. Also, strippers seem to like him.
The Problems: How exactly did Duke Nukem pull in all these babes in the first place? Forget it. Let's assume that in the Duke Nukem universe a terrorist madman poured a vat of Daddy Issues into the nation's water supply and thus good old Duke is considered charming to these women rather than a hybrid between somone's creepy nephew and a group of hormonal fourteen year olds' mab-libs game. Duke doesn't seem to value commitment, flipping from girl to girl, and occasionally taking a few potshots at them whenever he gets bored. Even though this won't hurt them, it certainly doesn't look good in court.
Oh, also, if you finally start to fall for him he'll disappear for over a decade, all the while promising to return, but not until you've moved on and when he finally does he'll be stumble in drunk, confused, and nothing like the fun, brash protagonist you remember.
Also, he probably has Chlamydia.