Well obviously on a sexuality compatible level, Blaine would have something to enjoy from Sam, as well, and Tina from Blaine. It’s not three sets of pairs it’s three people together.
What it’s sort of about is… it’s having those parts which are interesting sexually are definitely there for each of them in different ways. But, sexuality isn’t always black and white, sometimes it’s grey and it swims, or whatever Jane Lynch said.
It’s finding sex sexy, finding the idea that someone is getting off in part because of you sexy, even if the physical act isn’t so sexy to you? Enjoying pleasing someone. And it’s about a closeness between those people, them being a unit. And not being able to to envision doing anything like this without all of them, because they are a “friendly threesome” together. Taking that intimacy in a different direction. And trying something out that may or may not work.
Incompatible sexualities fic certainly may not work for some people, but it’s actually something I love, which is why I have read pretty much all the Kurt and/or Blaine + women fics where they aren’t bi that I have found, and have a half finished Kurt/Santana fic. It’s why I really wish someone would write me sexual Klaincedes.
I am marking posts and will mark the fic clearly (with multishipping and sexual blamtina), so that people who don’t want to read it can avoid it. It’s also not really supposed to be a realistic idea, much as I’m writing it in a relatively relatistic style. For all there’s loads of Klaine negotiation/making sure it’s fine, I cannot actually see that happening in canon at this time, but I’m bending what I can realistically see for this fic. Because it’s fic and that can be half the fun!