That is, their rampant, unabashed sexiness.
Already, I have to backpedal and clarify. By sexiness I do not mean the conventional sexiness oft associated with the comic book superhero genre. Rather than exploitative, hypersexual super(model)heroines, what I'm talking about is sex and sexuality as an explicit subject. Grant Morrison's New X-Men run was unashamedly about sex, even whilst his artistic collaborators, such as Frank Quitely or Igor Kordey, were depicting the usually glamorous superheroes as increasingly ugly and grotesque. Similarly, I'm not talking about some puerile, monogamous copulation between longtime sweethearts. The torrid, lustful, angst-driven sexual relations that truly define the X-Men against their contemporaries is what's of interest to me.
And we can see that this sexiness, this seX-factor, penetrates throughout the franchise's history. The aforementioned Morrison has seen the promotion of his work, as having a "back to basics" design philosophy, scoffed at since its publication (as it featured, amongst other things, a malevolent germ), but the one "basic" he did return to was a deep sexuality. Not only was the familiar Cyclops/Jean Grey/Wolverine love triangle returned, but at the forefront of it all were questions of virility and progeny; as mutants declared themselves humanity's successors as the latter marched towards extinction. Tensions around the sanctity of sexuality abound particularly in the conclusion of this story, where a doomed future is averted because Cyclops, rather than sustain his infantile and sexless relationship with childhood sweetheart Jean Grey, opts to pursue a relationship with his sex therapist, Emma Frost.
Emma Frost occurs perennially in discussions of seXuality. She was introduced in the most defining X-Men run, Chris Claremont's, and as part of the beloved Dark Phoenix Saga (soon to be adapted, for the second time, in 2019's Dark Phoenix). There's a popular misreading that this story is a cosmic one, as it has some light science-fiction features. The reality is it's a grounded story and a deeply sexual one, as BDSM-imagery seems to clutter half the pages and the story itself is explicitly concerned with a destructive female backlash at the removal of sexual license and her transformation into a fetish object. Claremont used sexuality in his stories with reckless abandon, spanning topics from female puberty, body autonomy and dimensions of consent, to commentating on the AIDS-epidemic. The follow-up to Dark Phoenix Saga, Days of Future Past, for example, is literally about sexual license, as mutants have their breeding rights prohibited in a dystopian future.
Jean Grey in the Dark Phoenix Saga.
So, Emma Frost was a character created in an entirely sexual context. Yet now, detached from the time of her creation, Emma Frost is often the centre of a lot of reader anxiety. What you often seem to have is a sexualised character, wearing revealing, exploitative costumes, who appears in stories that are actually not about sex or sexuality. It would seem that reader anxieties around Emma Frost's depiction are located in this arena; she is a character about sexuality who no longer possesses (or at least professes) sexual license. Emma Frost is devoid of sexuality, allowed the kind of innocuous relationship with Cyclops that Morrison intended her to disrupt. She is a character who is sexual in nothing but her aesthetic. In this sense she is a microcosm of the superhero genre on the whole, the male-dominated genre (both in terms of creatives, audience and characters) which is caught constantly negotiating and renegotiating its opinion on sex, reaching a point where sexuality is simultaneously hypervisible and invisible.
Emma Frost.
Going back even further, we can highlight that sex was actually important as early as the Lee/Kirby collaborations at the title's launch in the 60s. It was subtextual, of course, thanks to the Comics Code Authority and general sensibilities of the time, but its presence was there nonetheless. Whilst revisionist readings of the X-Men's early years still stick to the idea that the franchise had always been a Civil Rights allegory, the reality is that those stories owe much more to Cold War narratives and the emerging youth culture of America's post-war economic boom. This generational conflict rears its head again in the Morrisonian interpretation of the franchise and in both realisations sexuality is important. It is more overt in New X-Men, without a doubt, but those early stories still concerned themselves with virility and sexual license, particularly with regards to Cyclops and Jean Grey (and the other bumps and tensions the relationship met along the way. Explicit sex was off the table in the 60s, Jean and Cyclops don't even kiss until the series is revived in the 70s, but what we saw in those early issues were normal, well-behaved children with acceptable American values lusting after each other.
So, why does the return of a seX-factor matter? It's because, as a concept, it unifies the Morrisonian, Claremontian, and Silver Age renditions of the book. Three wildly different, and popular (although people only seem to enjoy the Silver Age stories, retroactively, it was not a popular book at the time), interpretations of the same characters and core concepts. Even with widely derided, "cursed" runs on these books, such as Chuck Austen's Uncanny X-Men or Peter Milligan's X-Men, that sexual element ensured some slight success, though perhaps they went too far in some places. (It is worth noting that Peter Milligan and Mike Allred's X-Force/X-Statix professed sexuality in droves- and is widely lauded, in part, because of it).
With a recent influx of fan creators carving X-Books into their own images, we are seeing, once again, a return towards stories prominently featuring sexuality. Sadly, though, these only superficially deal with the subject, allowing a voyeuristic pleasure for the reader but never allowing sexual themes to permeate the stories. Kelly Thompson's Rogue and Gambit was a great horny start, but there also needs to be a move away from the "beautiful people" dimension of sexuality and this is an area X-Men stories have usually done well in. Stacy X, the snake-skinned sex worker who had a brief stint on an X-Men team in the early 2000s. I think it's telling that Stacy X has mostly been scrubbed from the franchise history and, on the rare occasion she is acknowledged, her snake-like skin is nowhere to be seen. Do ugly mutants, the Eye-Boys, the Maggotts and Thumbelinas, not possess the same sexual license as the glamorous ones?
Stacy-X.
It's important, as well, to note that simply existing in a sexual dimension is not enough. Like any other story element, there must be a reason, some justification, for a story's sexuality, lest it become glorified, softcore pornography. We don't want to return to the past of creating female characters whose explicit purpose is to allure the sexual attention of 12 year olds, so creating male sexual objects, whilst an arguably egalitarian pursuit, doesn't actually solve the meta-sextual void that superhero comics finds themselves in. The superhero has long been a fetishistic concern; the genre (and the X-Men) cannot move forwards without acknowledging this inherent quality. Yet this should not be implemented by the cynic, who sees this as yet another opportunity to insert perverted "realism" to the superhero genre, or a chance to again put infeasibly attractive characters in revealing costumes. Rather, the re-sexualisation of X-Men should come about by creators who believe in sex, who do not hold our cultural pretensions around "acceptable" forms of love.