Sure. You don't think the implication was that Cupcake was attracted to Uhura, and that's why he interrupted Kirk's conversation with her? It's not an absolute given, which is why I put presumably in my initial post, but it's certainly the most likely motivation.
Hmm, I suppose that's a good point, but I didn't pick up that implication; I read it more as "Cupcake's a jackass that likes to fight and thinks townies are lesser than cadets" - him getting pissed at Jim hitting on Uhura was more to do with that sense of superiority and (maybe) watching out for his friend. I wasn't disagreeing that Cupcake almost certainly is hetero, I was just surprised you listed him so confidently.
And as I stated above, there's no reason why one of these characters can't have gay adoptive parents. I certainly don't want them to replace the characters that we already know, but there's plenty we don't know...
True enough re: Demora, but when Kirk says "it wouldn't be the Enterprise without a Sulu at the helm" that speaks more to genetic legacy and resonates as a biological relationship; she's a Sulu because she's a good pilot like her father, "naturally gifted" in a sense. Even that, if you wanted to, could be sci-fi'd to have Demora be the product of gay parents, but seems stretching.
I'd hardly call them bad examples. They are perfect examples of how pervasive heterosexuality is in these films. It was stated above that in a two hour movie there isn't time for such relationships, but that's patently false. We know about McCoy's based off of ten seconds of dialogue that didn't advance the plot but did get a laugh.
Displaying humans (and their genetically compatible extraterrestrial fellows) as the product typically of sexual biological reproduction is an example of pervasive heterosexuality? Sorry, I don't follow, especially as a core of Spock's identity is his divided biological nature.
Something like McCoy's is a good point, easily established as one character asking a male character "how's your husband?" as an aside with no special attention drawn to it.