This entry is reserved for an entry that I'm too busy to write at the moment, on the subject of Stiaan and relationships. Hopefully this placeholder will bloom into an essay by midweek; if it hasn't, someone should poke me. XD Right, have time now.
So, Stiaan has a boyfriend. Hence, as is my custom, I essay on the factors which led up to this. It will wander for a ways before it gets to the point, most likely, as is also my custom.
Stiaan and Kastor
I don't ship this pairing...well, okay. I don't ship it as things stand in canon. I think that Jesse's done far, far too good a job as portraying Stiaan and Kastor as growing a friendship, not a romance. They don't behave like they'd even consider sleeping with each other; they both dismiss the notion out of hand, explicitly. Now, this IS Jesse, and he only tells one story, and so somehow he will contrive to glue Kastor and Stiaan together eventually. But as it stands, any kind of romance would end badly and probably very quickly, and they both know it. This would be the case even if Mikah wasn't in camp, and given that he is, Stiaan would cut off an arm before he denied those two any of the happiness they could find here.
Stiaan sees Kastor as a friend, definitely, and will admit that he's attractive, but foremost in his mind is always the wrong he's done Kastor by being the cause of Mikah's death. The fact that Mikah is here in camp in no way mitigates this, although it does wonders for him in other ways. (Sorry, taa.) What Kastor HAS done for him is wear down certain prejudices, and get him used to dealing with mortals as equals.
Stiaan and Mortality
I don't think that Stiaan has had a lot of sustained contact with mortal folk over his lifetime. Mikah's the social one, and Stiaan preferred to close himself away doing research. Until he got his ass kicked by the party in the first book, I seriously doubt it ever occurred to him to seek companionship anywhere other than Mikah -- who else would he have needed, after all?
The aftermath of Mikah's death changed all that. Stiaan's worldview, self-image and self-worth basically shattered, and he's had to piece it back together. Along the way, he got shocked out of his previous view on mortals and mortality, and started looking at everyone as individuals. You can see this with the way he treats Charis, with the way he interacts with Kastor, and during his fight with Rema.
In canon, Stiaan says several times that he thinks taking mortal lovers is in bad taste. I, however, have reason to believe that the gentleman protests too much. He's always referring to Kastor somehow when he brings it up, and often it's in context of dreams being sent to him by the gods, which is sort of distasteful all on its own. The above section, I think, demonstrates why Stiaan has reason not admit to being attracted to Kastor, but he also doesn't want to be personally rude or bring up Mikah if he can help it at all. (Stiaan is only sometimes blunt and honest.) So he clings to a prejudice against mortals as an excuse more than anything else, and all the while that prejudice is being torn down by sustained contact, as most prejudices tend to be torn down.
Stiaan and Edward
So we have Stiaan, who is undergoing a personal attitude revelation and losing all his prejudices, thrown into camp, which makes the process go even faster. He's interacting with a lot of mortals, and they're all interesting, and he's enjoying it. Mikah's alive and around and forgives him, so he isn't driving himself into exhaustion trying to atone or angsting that he's not capable of atoning in camp. Kastor's around and still friendly and he and Mikah are still together. And this, too, affects Stiaan's worldview -- there's a mortal making his brother happy.
When he met Edward, they hit it off almost immediately. Edward is intelligent and fun to talk to and kind to him, and they could bond over shared experiences against a/the gods (and over having made mistakes in the past they're trying to atone for, among other things.) Edward pinged Stiaan's "I want to be friends with this person" triggers very hard; they share interests, they're easy in each other's company, they have the same dry sense of humor, their social dysfunctions are compatible ("awkward and not really understanding things, but trying hard") and Edward doesn't seem to care about the fact that Stiaan is immortal and ridiculously powerful. It doesn't hurt that Stiaan has a type, and Edward is it (swordsman, dark hair, intelligent, literate and capable). But Stiaan didn't (still doesn't, really) think that he deserves to be cared about, as much as he wants to be, so he never expected that Edward would be attracted to him.
As it turns out, he was kind of blindsided, and by that point, Edward had become one of his most important people. He had no need to hold back out of guilt for past actions, a very healthy dose of his own attraction, his prejudices were pretty much gone (and possibly gone for some time) and his best friend (probably about equal to Kastor at this poing), who he'd come to care a lot about and knew was attracted to men because he'd talked him through the process of accepting it, was making his attraction to Stiaan very obvious. Edward offers him a chance at a kind of...well, as normal as a relationship can get; something that's based on mutual compatibility, attraction and caring. And Stiaan is very, very vulnerable to people caring about him or valuing him -- he doesn't think his existence has value or worth, but he's completely topped by people who disagree. So he admitted that he was attracted to Edward, too, and things proceeded from there. And things have continued to proceed, to the point where both of them are ridiculously happy and dentist-defyingly cute.
Hopefully this sheds a little bit of insight on how Stiaan's brain was working during this whole process; if people have comments or want clarification, please feel free to ask.