When I was working on Mistborn 2 with my editor, he asked me, "Are Vin and Elend sleeping together?" I said, "Absolutely." He requested some confirmation of it on the page, and I explained something that has always been my policy, and one that has served me well.
I consider what I'm writing to be a very detailed script, which you the reader direct in your mind. Each person's version of the books will be slightly different, but in sometimes telling ways. The subtext of conversations will change, the visualizations of the characters, even larger implications are changed, distorted, and played with by the reader as they build the story in their imagination.
This is an area in which I prefer to leave the answers to the reader. For those who wish to imagine that the characters are having sex, then the implications are often there. (Though I've gotten better at that balance, I feel.) For those who don't want to imagine it, and wish to pretend the characters are living different standards, I will often leave the opportunity for that--unless it is a plot point I consider relevant.
Certainly, my upbringing and beliefs are an influence on this. I'm obviously more circumspect in these areas than I am in others.
But yes, for those who don't want to pretend otherwise, Vin and Elend were sleeping together. And Wax and Lessie never had a real ceremony. My editor tried to remove the word "wife" from one of the later books, and I insisted, as the shift in Wax's thinking was a deliberate point on my part--related to his changing psychology in the books. But even to him, it's more a 'common law wife' thing.
As a side note you'll likely find amusing, I do get a surprising number of emails from people who complain to me (even take me to task) for the amount of objectionable material I include in my books, and ask me why I have to wallow in filth as much as I do. I'm always bemused by this, as I doubt they have any idea how the books are perceived in this area by the general fantasy reading world...
Does this mean that Wayne and MeLaan's fling is "a plot point [you] consider relevant"?
Calling it right now, Wayne's... intimate... knowledge of Kandra biology will be a point on which the fate of the entire cosmere hinges. Because why wouldn't it.
The plot point isn't exactly what you think it is, but yes.
One of Wayne's roles is that of a character who points out absurdity, either through word or action. There is a certain level of absurdity in what I described up above, and I realize that. Some things I talk about explicitly in books, some things I don't.
On a certain level, Wayne showing that people do--yes indeed--actually have (and talk about) sex in Sanderson books is there for the same reason that a court jester could mock the king. When as a writer you notice you're doing something consistently, even if you decide you like the thing that you're doing, I feel it's a good idea to add a contrast somewhere in the stories.
It's one of the reasons that Hoid, though a very different kind of character from Wayne, has more leeway in what he says in Stormlight.
I know this was a few months ago, but I have a follow up question (huge fan of your work btw!): Do you purposely mention characters having sex to show that they are maybe not "good guys"/"bad guys" are mentioned having sex as a continuation of their lowered morals? Like OP mentioned with rape, of course that would be a sign that someone is a terrible person, but I can think of several other instances in your books were someone engages in consensual sex who later turns out to be more morally loose.
ETA: I mean premarital sex
I don't personally consider this to be a sign of who is good or bad, but I can't speak for how the morals that shape my own society might affect my unconscious application of morals in my books. That's certainly something for critics to analyze, not for me to speak on.
If it's relevant, though, I don't perceive it this way. More, the people I mention engaging in premarital sex are ones more likely to reject societal mores. (Such as MeLaan.) I also am more likely to do it for characters who are not primary viewpoint characters, for reasons I've mentioned--the ability to allow plausible deniability for readers who wish to view the characters in a certain way. I can see myself unconsciously letting myself say more about villains for a similar reason, though I don't intend it to be causal.