Isn’t he married?

First, Electra Complex my ass.
 
Second, Fancypants didn’t mind all the attention she gave him at all, so there may be some degree of fondness. The mane flip was basically her being absorted in her constant modelling, and it’s a bit unfair to judge her on only one line (which in my opinion didn’t even sound dumb, she was just commenting on Rarity’s bags).
 
She did appear for a brief moment during Rarity’s song, and it’s not like she has to appear besides him in every scene, her job may have got in the way.
 
I think that him having a mentor role fits him the best. He was nice to Rarity, but there was no hint that he was interested in her romantically (and knowing Rarity, she would have shown some). The relationship was more like her admiring him, and him seeing the potential in her, both in a professional way.

@WhiteKnight  
tl;dr I could see it either way

Idk, could work.  
I dont see Fancy Pants as being significantly older, probably acts older.
 
And Rarity seems to have an Electra Complex anyway.
 
As for Fleur. Well Fleur can be seen as trying to get FP’s attention physically. She even does a mane flip but he walks off without her. The fact she is never seen again says a lot also.  
And she sounded kind of dumb when she spoke, so I prematurely peg her as a socialite who gets by on her looks or Dad’s money.  
Whereas Rarity could get by on her looks if she wanted but doesn’t.
 
But at the same time she is a unique flash model. Maybe her presence is to prevent the ship and establish the relationship between Fancy and Rarity as platonic, or show that Fancy is looking past the physical with Rarity.  
I honestly wouldnt be surprised if Rarity is supposed to of comparative beauty to Fleur despite using the same flash model as the other mane 6. Or if Fleur is what Rarity looks like when she gets older.  
It wouldnt make much sense but at the same time a lot of things dont make sense and we’ve never seen a pony look anything like Fleur who wasn’t also an Alicorn.
 
I could also see Fancy in a mentor role, seeing a lot of himself in the young white unicorn.
 
There’s an upcoming episode called “Sleepless in Ponyville”, named after a famous RomCom so that may settle it.

Also, Fleur. After all those caresses it’s hard to think of her as anything other than his wife.

I could never get behind this ship because Fancy Pants is atleast the same age as her father, if not older.

He striked me as in his early-to-mid 40’s.

How old is Fancy Pants?

Katara?