Part 1

Spitfire as a seductress is becoming so prominent on a lot of her fan depictions that it's becoming her main fanon trait.
It's in contrast to the show's depiction of her as being primarily just an professional athlete.
That defining trait of hers is becoming more and more secondary in nature within the fandom.
This is more than just introducing sexuality to her character.
No longer is she the legendary heroine that Rainbow Dash admired but is now devolving into something lewd.
Though it won't likely change Spitfire's characterization within the show itself (if she returns to an active role that is),
it certainly will change how the general brony population will perceive her as (and ultimately depict her more often as).

It's similar to the phenomenon of Princess Celestia being perceived as an epic troll and a molester within the fandom.
It's almost guaranteed for you to see the words "troll" and "molest" in the comments sections of anything she appears in.
But unlike Celestia, Spitfire doesn't have much characterization within the show to speak of so she is more affected by these trends than Celestia is.
Almost in the same level as a background pony's personality and back story being fleshed out and becoming a generally accepted fanon that will define them for the rest of the fandom's lifespan (or until a canon event happens that will destroy it).

@Darque
I totally understand and agree with your point but:

That is in the actual show, so sexuality is addressed (though not explicitly obviously) through Rarity.

As for Spitfire, I'm pretty sure her sexualization began as a sort of Sports Illustrated pin-up and has evolved from there.
Her general style and attitude seem to actually fit this type of art better than a lot of others, but that's open to opinion.
In the end, it all comes down to what the person feels like drawing.
If I like it, cool.
If not, I move on.