No, it's not.
It... is hard for them to feel romantic feelings, actually. After augmentations, the sex drive was suppressed. Did you forget that? The nature and environment of Black Team is rather different than the nature and environment of Blue Team. If I recall incorrectly, wasn't Margaret and Otto's relationship only before augmentations anyways? That reasoning simply doesn't work since we have multitudes of evidence that suggest Blue Team is a family. They view each other as siblings.
No, it's not "common sense". You do realize that people who are your friends can still be considered a part of your proverbial family, right? Also no it's not hard for them to feel romantic feelings, and we know this because of what we saw Victor, Otto, and Margaret go through in 'Halo: Bloodlines'. And as I've pointed out many times already, there is nothing in canon that indicates John and Kelly view each other as "siblings" in the literal sense. Ergo at this point they are no different than any childhood sweethearts who have stayed close their entire lives.
It's common sense. It shouldn't ever happen. Blue Team sees each other as a family, not as friends. It's also rather hard for them to feel romantic feelings. Promoting a John-Kelly relationship is akin to practically promoting incest.
And you're going to back this up with anything substantial, or...?
Well, if they 'might', then it'd be a catastrophically written story. It won't be happening.
A possibility that won't come to pass.It might
Seriously. Why is two people who have cared about each other their entire lives, who trust, and respect each other in healthy ways possibly becoming a couple so scary? Especially if it's done the same way their relationship has been built up in things like the books up to this point? The execution of it is obviously important, but unless you can see into the future with 100% accuracy there is no declaring how good/bad said story would be written.
The catalytic thyroid implant the S-IIs got only had the potential risk of a reduced sex drive - i.e. such a side effect was never guaranteed to happen at all or be permanent if it did. Not that it matters anyway, as sex drive has absolutely no bearing whatsoever on a person's capacity to feel genuine love and desire companionship. Also in what ways is the "nature and environment" of Black Team at all different than Blue Team? There literally is none.
They went through all the same training and fought in the same wars. And while it is true that Margaret and Otto's relationship was while they were in training, it is made clear in the comic that Victor's own unrequited affection for her remained into adulthood. So it stands to reason that even after the augmentations Margaret and Otto could have maintained their relationship if not for Victor's interference in the first place. Not to mention we have Randall as another example; he fell in love, settled down, and even had a child with his wife after being stranded on an abandoned colony world well before the UNSC relocated him and eventually required he have most of his augmentations reversed as grounds for his release from service. Also you're confusing the meanings of the words of "family" and "relatives", as there is a difference between the two. Your close friends and significant other can be just as much a part of your family
as anyone related
to you by blood, if not more so. Now, some
Spartan-IIs seeing each other more like literal siblings is bound to be the case for certain individuals - but again, obviously that isn't the case for all
of them. And since we have yet to see anything that proves John or Kelly equate each other to siblings, they still fall well within the realm of individuals like Margaret, Otto, and Victor.