Rarepair shipper here, and I think it mostly depends on the place the character has in the story. Twilight is a main character, Marble isn’t. I’d call it fair.
 
Too bad Marblemac sunk a ways back and Pony!Twilight hasn’t interacted with Flash Sentry in god knows how long, though I probably missed something

Yeah, maybe cousins. And very very distant possible cousins at that. There’s a point in genetics where two people share a common ancestor, but through the generations, their genetics have become so different that they are, from a genetic and legal standpoint, not related.

“Fourth cousin twice removed by a fifth cousin.” Possibly maybe.  
If that’s too closely related, I don’t recommend dating anyone born in the same town as you.

>memes  
Pack it up

Okay, I see this keeps going on and on, so here’s the thing.
 
The Marble Pie-Big Mac thing is a cute little gag that was done in a funny/awkward way, not forced, just random, and the fandom liked it. It wasn’t obnoxious, and it was never addressed again. Also, it didn’t mean they were attracted to each other, they were just uncomfortable, but the fandom interpreted it like love (we tend to ship ponies with goddamn plastic straws, it’s not weird we started shipping them).
 
On the other hand, the Twilight Sparkle-Flash Sentry thing felt forced as fuck since the first time it was done in the first movie (the worst and most forced one out of the three movies), and it wasn’t funny, not even meaningful to the plot, but they forced it down our throats every time they had the chance anyways.
 
“But dude! It’s a teenagers-go-to-high-school-and-do-teenager-stuff movie/musical! And a cartoon one nonetheless!”
 
Oh fuck you, that’s not an excuse, even fucking High School Musical did it better.
 
Also, “[…]it doesn’t matter that they are cousins[…]”
 
The last time I checked, fourth-cousin-twice-removed-by-a-fifth-cousin genetics related me with Daniel Radcliffe and Kim Jong-Un.
 
tl;dr: OP was raised in a duck farm

@Starswirl  
Eh, morality and emotions work a bit differently in that universe, what with unicorns being above such things. It’s only once she turns into a human that those emotions have an effect on her, which is specifically because she’s become so human. In Twilight’s case, ponies are clearly capable of love; she just happened to fall for Flash due to plot reasons.
 
Not making a case for Flashlight, by the way, just stating the differences.

@Background Pony #8C50  
She was human at the time they developed a crush on one another. I’m guessing The Last Unicorn is also icky to you?

@Background Pony #8C50  
Just adding this because i have a doubt,should i have spoilered zoophilia ?.That’s a genuine question,i dont know if it break the “image ratings rule”

Let me simplify this to you people  
Mac and Marble Pie are both ponies,it works,it’s simple,it kinda make sense  
Flash and Twilight,one is a pony,the other is the alternate human version of a pony.Convolutated/confusing/awful writing aside,the final nail in the coffin of TwiFlash is the delicate relent of zoophilia that come from this pairing (a pony fall in love with a human ? Go back to FimFiction,Hasbro suits)

To be fair, Big Mac and Marble actively got to know each other over several days. Twilight and Flash bumped into each other a few times but otherwise knew virtually nothing about one another.

@Spritejunkee  
Indeed. Even then it’s going to be tricky, because so many people are going to have their own ideas as to who they should be with, and you’re never going to please them all.
 
But I’m pretty sure you’d please a lot more of them if you make the love interest someone who actually existed previously >.>
 
Again, secondary characters have less of an issue with that.

@DoesNotExist  
Yeah, the shows main character having a love interest can be difficult unless it’s with another character.

@Joseph Raszagal  
I’m not saying that Marble is more developed, or that she has a deeper character than Flash. Hell, Flash has more character now by dint of appearing in three movies.
 
What I’m saying is that Marble wasn’t created for the purpose of being a love interest, and that her ‘relationship’ (it admittedly wasn’t much, mutual blushing that implies mutual attraction and not much else) actually served the story as opposed to existing for the purpose of attracting a new audience.
 
For clarity’s sake, the argument is not that she’s a better character for having existed, but it’s easier to accept her as a love interest because she was pre-existing, even in as tiny a capacity as she was.
 
And, as I’ve said before, she’s a love interest for a secondary character who also doesn’t have that much of a character built up. That’s far less of a big deal to fans than a character who’s the love interest to the show’s arguable main character.

@DoesNotExist  
The fact that Marble existed does not make her more developed, it only means that she existed.
 
Otherwise, that’s like saying you know who I am as a person by virtue of knowing that I exist. That’s ridiculous, every single kind of ridiculous.
 
Besides, within the same parameters, both Marble and Flash check one of the same boxes (which, admittedly, does depress me). While Marble “existed” prior to her first real episode, she still got used as a possible love interest for a preexisting character and did literally nothing else.
 
Again, keep in mind, I’m not supporting Flash here. He’s bland and exists solely to make human Twilight blush. That’s it. However, arguing that Marble is more of a character is just barking up the wrong tree. I’d rather wait for an episode to flesh her out more than just throw character traits onto her that aren’t actually there to begin with.
 
She’s shy, she says “Mmhmm”, and she blushed when pushed by Pinkie into Big Mac’s face: Thus far, that’s her entire character.

Hypocrisy is a fun thing.

@DoesNotExist  
That’s basically it.
 
Twilight x Flash is “well developed main character with a lot of depth” getting paired with “random nobody that came out of nowhere and suddenly is the hearthrob for main character with no character definition”
 
Mac x Maud is “vaguely fleshed out side character” getting paired with “only previously mentioned yet also vaguely fleshed out side character”.

@Joseph Raszagal  
Ah, but Marble still existed before the episode in question.  
She wasn’t created to be a love interest specifically, unlike Flash. That’s one of the reasons people are inclined to reject him, because he springs out of nowhere for the primary purpose of being a love interest, presumably to attract the tween/young teen audience.
 
Marble on the other hand became a vague love interest after already existing as a character, and for reasons that arguably better served the plot. All right, she didn’t have very much depth to her, but the sheer fact that she wasn’t created to be a love interest makes it feel more natural.
 
Now you could say that’s irrational, sure, but I don’t think it makes this hypocritical. After all Marble is being pushed with a character who’s a secondary character at best, the conditions for depth etcetera aren’t as strict as they would be for a character who’s being pushed with the main character of the show.

@Daneasaur  
Dunno, yo.
 
This much I can easily work with: Flash is certainly a flat character in that his entire existence is based around the fact that he’s nothing more than a love interest for Twilight to fawn over. That’s definitely true.
 
However, I can’t in good faith say that Marble is in any way a fleshed-out character in her own right. Like Flash, she pretty much exists as a satellite character, the difference being that she’s a sister for Pinkie as opposed to a romantic interest. She’s important because she’s Pinkie’s sister, not because she has rounded and developed traits. The two things that one could argue are “established” about her are her relationship with Pinkie, which wasn’t built upon in the episode, and the fact that she lives on a rock farm, which wasn’t built upon in the episode. Really, I could argue that Limestone is a more developed character, and all we really know about her is that she’s really pissed-off most of the time.
 
Marble matches well with Big Mac in exactly the same way that Fluttershy does; she’s shy and quiet… that’s all we know about her. Marble is essentially a nice, cold can of Diet Fluttershy.

@Techzom  
Bingo.
 
Flash has no real character other than “I’m supposed to be a romance for the lead lady.”
 
Marble Pie, on the other hoof, is established along with her sisters and, in fact, seems to mirror big mac in a lot of ways. Her reaction to mac is the same as mac’s reaction to her.
 
She also existed prior to her appearance in this episode, so she didn’t just pop out of the ground.

You know, OP, as much as I love shipping for the sake of shipping, I do agree with ya.

@Rpground  
I’m fine with that.

@Background Pony #CE2F  
no shit… tell me something new

@KillerKid86  
The only word to describe that is “ignorance”.
 
If you want to be ignorant to facts, then I guess we’re done here.

@Rpground  

 
Dude,not to sound mean but I really don’t about other peoples options anymore. People hates Flash, I don’t.
 
People like Marblemac, I don’t.
 
I’ve stopped caring about what others think and only focus on what I think.
 
Close minded yeah. But at this point in time; I can’t be bothered to care.