It's obvious I've entered this discussion far too late and I hope I don't repeat anyone, so here goes...
I agree that the level of (or lack of) sex, or sexuality, is all but non existent in this series, but I never gave it much thought. I cared more about whether or not Vin and Elend would BE together, which, in Vin's point of view, was frequently tossed back and forth. That's a believable characteristic for a young female character. I can believe that Vin is more concerned about "being together" over "having sex". It's not far fetched from the male perspective, which is the other way around, stereotypically speaking. You have lots of gender difference data to back that up.
I'll come out straight but saying, yes, I'm Mormon, grew up in Burbank California (close to WB and Disney Studios), now working/writing in Utah. I don't have any qualms talking about sex, if the subject of discussion calls for it. It would be random if I brought up the topic of sex in the middle of a conversation about a worm in an apple. If someone introduced the unconventional reproductive nature of worms, than I'm free to open up a "can-of-worms" so to speak
I recall a novel, Benford's
In the Ocean of Night, where the protagonist is in a laboratory, a problem is introduced, page break, and the last page or so of that first/second chapter has a mildly descriptive course of physical actions (sex) taking place between three people. Wait... that was kinda randomly put there, I thought. I kept reading a little more and it turns out that scene did nothing for the characters, the plot, or resolving the problem. I ultimately put it down, and that's the only time I've ever put a book down. That's coming from a Mormon whose read Hannibal/Expendable/Lolita and enjoyed them. As a writer myself, not published but still young and learning, I wouldn't use sex or sexuality in a story unless it calls for it or if it helps develop character, plot, and resolution. As an extreme example, I really love the Terminator story. What appears to have been a random romp between Sarah and her protector from the future ultimately results in the consequence of Sarah becoming the mother of the child who grows up to lead a band of human survivors in a resistance against the machines. Very Messianic if you think about it...
Anyway, I didn't have a problem with a lack of sexuality in the story, and, as a student of Psychology, I can see some reasons for the character's lack of sexual interest, now that the topic's been brought to light. Granted, I'm halfway done with HoA , so this may not include everything...
Hammond: He's married, for one, so its very "noble" of him to be faithful to her and their children.
Breeze: He's practically a cradle-robber in WoA. That made me laugh for a while.
Sazed: Being a eunuch, he's frequently embarrassed by the fact that he can't do anything, though it was subtly noted that the desire was there, more so when Tindwyl was around.
Vin: her paranoia of betrayal is keeping her from opening herself up to the next step in the relationship.
Elend: His father forced him to "bed" a skaa woman at 13 and later found out she was killed. This trauma did a few things for him: 1) It disgusted him so much that he didn't want to do that again. 2) He respected women more than most, not seeing them as objects to be played with, however, he could not stand the women of the court because they shared similar views as his father, so he didn't "look" at them. The fact that he managed to single out Vin so quickly tells you how that bad experience matured him. 3) It caused him to think of his nobility differently, creating a desire to "make things better".
Spook: Always jealous of Elend because he likes Vin, which entails he thinks of her still, a lot, even though she has a "boyfriend", which implies desire (a very common young male attribute. Been there).
That's my undergrad psychoanalysis for ya
Overall: The scope of the problems they are facing. As a young man now, if I were in a position of power with three enemy armies around my city, I would be more concerned about being killed/running the city than engaging in sensual/sexual activities too, but that's me.
I find the series refreshing in that these subtleties are woven in the story, providing intrigue throughout, but the problems needing to be resolved are... unthinkable, unwishable. As a reader, I want to know what happens next, and soon. For this story, putting more sensual/sexual thoughts/themes into it is a distraction from the action to a different kind of "action". Some of my friends who are not Mormon, who've since read Mistborn, gave no thought that it was written by a Mormon author until I told them. Of course, the scholars that they are, like myself, they picked at it deeper to find moralistic consistencies that coincide with that faith. Mistborn is a moral story. Destroy immorality (Lord Ruler) and learn again what morals are, what faith is, what believing is. Though not yet published, I can already tell you that my adult age group novels have more sensual/sexual implications than Mistborn does, but not that much more, since I'd rather tell the story than get into heavy, less urgent details.
Fun question. Thanks for posting. Got my brain working for today