From the source:
 
“Oh not a problem at all Twilight, say would join me in the little mare’s car?” She asked politely, as she hooked one of Twilight’s forelegs and immediately yanked her roughly from her seat and began dragging her out of the private cabin, down the hall and tossed her roughly into the mare’s room after which she shut the door. Once inside the very tight confines of the bathroom built for one at a time, Rarity spun to face Twilight. The two were so close that their muzzles touched.
 
“Ow!” Twilight complained as her horn bumped in into Rarity’s “What in Equestria are you doing?”
 
“WHAT IN EQUESTRIA ARE YOU DOING?!” Rarity unloaded. “Do you realize the hoops that I’ve had to jump threw to arrange this?! Are you even thinking about your future?!”
 
“What are you talking about?” Twilight rebutted.
 
“Courting, Twilight, I’m talking about courting!” Rarity fumed. “After your Heart’s and Hooves Day fiasco, I wrote the Princess directly about your little problem and together we’ve created the perfect solution that you seem to be completely intent on ruining!”
 
“What fiasco? What plan? What are you talking about?!”
 
“I’m talking about your inability to handle yourself around stallions. That you are simply incapable of landing a date no matter what you do, however no I am beginning to see why!”
 
“WHAT?!”
 
“Don’t ‘WHAT?!’ me, Twilight Sparkle. You’re a dear and close friend, and as your dear and close friend, I’m telling you to open your eyes, and subsequently start batting them at that handsome colt sitting across from you!”
 
“Flash?”
 
“No, the Chineigh teacups on the table, OF COURSE FLASH!” Rarity bellowed. “It was bad enough you practically vomited at the idea of him spending the night with you at the Crystal Palace, which, by the way, any mare in her right mind would have promptly jumped at and then jumped on top of him! But then you’ve been ignoring him this whole way home, and you think it a grand inconvenience that he’s going to stay with you for a week!”
 
“Okay first of all, ewww! Second of all, I would have liked to know about that first, and it is an inconvenience, Luna has the guest room, so now I’ve got a Captain of the Royal Guard that’s going to have crash on my couch for a week!”
 
“You are joking right?”
 
“Well where else is he going to sleep?”
 
Rarity stared hard at Twilight.
 
“Oh No,” Twilight quickly spat out as she shook her head, “No, no, no! Not gonna happen.”
 
“Oh and dear, a bit of advice for you when it comes to entertaining stallions.” Rarity added once Twilight got the hint. “Instead of prattling on about Starswirl the Bearded, I suggest you instead put that mare’s mouth of yours to, how do I put this lightly, ‘proper use.’” She said candidly as she raised her eyebrows to emphasize her point.
 
“You’ve got to be kidding me,” Twilight groaned and she turned and banged her head against the bathroom wall in futility at what she was hearing, “He’s not gonna be sleeping in my bed. He’s not gonna be sleeping with me. And I am fairly certain that is NOT the ‘proper use’ of a mare’s mouth!”
 
“I know quite a few stallions who would beg to differ,” Rarity chuckled.
 
“You’re worse than Luna, you know that?” Twilight

@Exedrus  
At first I thought Twilight wants to hang a framed pictured on the wall, but couldn’t get the nail in the wall without damaging the wall in the process and Rarity laughing at her being all like  
“You can’t even hang a picture on the wall without Spike helping you?”  
and Twilight all like  
“Coming from the mare who has the repair mane’s private phone number…”

@Background Pony #5A6F
 
MY BELOVED ICE CREAM BAR. HOW I LOVE TO LICK YOUR BACON CENTER.

@Background Pony #F818  
Good to know I’m not the only one who thought that…

@amess
 
I MADE IT MYSELF!!!

Twilight: “WHY WAS THERE BACCON IN THE SOAP!”

What are you doing darling?
 
I’m kickin’ my ass! DO YOU MIND?!

@The Smiling Pony  
It’s a little more nuanced than that:  
Rarity is trying to get Twilight into dating (apparently without informing Twilight), but is baffled at her inability to woo any stallions. So, during her most recent attempt, Rarity tries to get Twilight and Flash together. As this slowly fails, Rarity drags Twilight into the bathroom to bemoan how bad things are going. Twilight doesn’t get it at first, which forces Rarity to be blunt. The marshmallow pony about tells Twilight she should be sleeping with Flash because any mare would if they could. Twilight, who has made it fairly apparent she isn’t into Flash, argues the point and eventually bangs her head against the wall in frustration as Rarity prattles on various innuendos (like the “‘proper use’ of a mare’s mouth”, etc).

The image source explains the whole scene. This made me laugh. Rarity is being very straight with Twilight.

@The Smiling Pony  
Aww, that’s not as homoerotic as I was hoping it would be.

@The Smiling Pony  
(Also, why do people like to make Rarity out to be a sort of ‘concubine’ of sorts?)

@The Smiling Pony  
I agree with Twilight. Based on that scene, yeah, Twilight’s kind of in the right for not wanting to bang a stallion she isn’t in a relationship with.

@The Smiling Pony
 
I would, too, if someone badgered me into starting a relationship when I don’t want to.

@Peng  
Cliff Notes from what’s in the source, Rarity wants Twilight to get it on with Flash, Twilight (angrily) tells her to piss off.

“R-Rarity?”
 
“You’re so cute when you’re flustered…”
 
“Why are you acting like this all of the sudden!? Just a second ago we were drinking punch!”
 
Outside, Rainbow Dash snickered as she hid a love potion beneath her wings

Did… Did Rarity poop on the wall?

“Back off, Rarity! This is my peeping hole!”

I would really like to know the back story to what is happening right now.