Straight characters have only one RO in this game: Breden. As I’ve said elsewhere, apologies, but I’ve written a rebellion that is an extremely poor dating service.
There’s no “being straight later on.” Your choice when you first encounter Breden determines whether you’re attracted to men, women, or neither. The choice of “both” comes later, when you meet Kalt/a or Simon/Suzane. It is possible with a bit of squinting to ignore that, as @Ramidel says, and the author’s intent is binding on no one.
If you got the grayed-out option, it’s because you picked “I’ve never been attracted to anybody that way” with Breden. For readers who have asserted that their MC is asexual, I didn’t want a choice later on that basically asks, “did you really mean it?”
My primary intent with K and S was to have characters of the MC’s non-preferred gender who are in unrequited love with them. That plays into the deeper themes of the game when it comes to questioning what’s natural and unnatural. I didn’t want to exclude bi characters, so K/S also had to be romantic options… but only for bi MCs.