The reason Doc Smith couldn't publish a sequel to "Children of the Lens" is because it had incest as a major plot point.
The kids were special in another way, too, he [Kit] had noticed lately, without paying it any particular attention... They didn't feel like other girls. After dancing with one of them, other girls felt like robots made out of putty. Their flesh was different. It was firmer, finer, infinitely more responsive. Each individual cell seemed to be endowed with a flashing, sparkling life; a life which, interlinking with that of one of his own cells, made their bodies as intimately one as were their perfectly synchronized minds.
[Mentor] "Your lives will be immeasurably fuller, higher, greater than any heretofore known in this universe. As your capabilities increase, you will find that you will no longer care for the society of entities less capable than your own."
Considering the Children are genetically perfect and arguably aren't even human anymore, this isn't as bizarre or offensive as it sounds.
Still, Heinlein, himself no stranger to incest-themed stories, wrote that Smith intended a continuation to Children of the Lens. In fact, there are even some textual clues in Children a sequel was planned: Christopher Kinnison delivers a flask of force, and it's not clear who finds it, and he states that Civilization is again threatened and that he is just a youth and not entirely up to the challenge.
Heinlein said in "Larger than Life" that Smith's Lensman sequel was "unpublishable" at the time. Could it have been because of incest-related themes?
The maddening thing is, we may never know. Smith told Heinlein what his Lensman sequel would be like, but we've never found even a manuscript or outline, and now both men are dead.
Like Heinlein said, "that's his story to tell. You must find your own."
Clark Ashton Smith's "Vulthoom" is set in the Lensverse
Pulp aficionados love to go crazy and guess which stories cross over, and it helps that reoccurring tropes make a lot of pulp stories very similar (like how most pulp heroes have gray eyes…including Kimball Kinnison, incidentally). For instance, one of my favorite pulp interconnections is that the scarecrow-bodied geologist-archeologist explorer main character in Lovecraft's "At the Mountains of Madness" is really Johnny from Doc Savage.
"Vulthoom" is a science fiction horror story where the primary villain is a tremendously long-lived, mostly unseen, hideously different alien being who is considered the Martian devil. Vulthoom's modus operandi is to operate at the center of a conspiracy with humanoid servitors that never see him directly, who he manipulates by using their self-interest. He has tremendous mind powers and is capable of conjuring psychic illusions. What's more, his chief weapon is exotic Martian narcotics, which he distributes and uses to control others.
Doesn't this sound like Boskone's modus operandi to a T?
Vulthoom even has a secret base hidden deep underground below Mars (shades of Helmuth's base!), where he is constructing a fleet of interstellar warships unseen by law enforcement.
The Mars seen in Vulthoom is consistent/identical with what we know about Mars from Triplanetary: a long-time Earth ally with a tremendously long, unknown history, a dying ancient culture. Earthmen visit often, but we don't really understand the inscrutable Martians.
The Alhai, giant and monstrous with withered faces, are not exactly biologically similar to Martians in the Lensverse, but that's actually a plot point: in the story that they are bigger and genetically altered and even the main characters acknowledge they look nothing like normal Martians.
Oh, and by the way: the hotel where earthmen stay is called the Tellurian Hotel, incidentally, with Tellurian being the Lensverse name for Earthmen.
Yes, women CAN use the Lens.
"Your report is neither conclusive nor complete."
One thing should be pointed out: Virgillia Simms' explanation for why she didn't get a Lens in "First Lensman" was her interpretation of what happened, not something she was repeating verbatim from Mentor.
Nevermind we SAW women use a Lens: Clarissa, in Second Stage Lensmen. In the David Kyle continuation novels, another woman becomes a Lensman: Lalla Kallatra. Kyle stated women could use the Lens, but they were rare to the point of being unheard of.
In short, there's no conclusive proof there are no women Lensmen, but we have several right in front of us. The belief they aren't is a "Black Swan" fallacy: because all of the swans we see so far are white, there can't be a black swan.
The belief women can't be Lensmen based on Virgillia's speech reminds me of how, for a long time, there was an attitude women couldn't be starship captains in Star Trek because of Janet Lester's belief in "Turnabout Intruder." Why are we accepting here at face value the point of view of a mentally disturbed, crazy person consumed by envy?