I would disagree, every marriage depicted in-universe was heterosexual, every relationship the same. There was never even a hint of a actual gay character.
Then explain the lack of gays in the future.
How many gay people did I pass when I walking down the city street just now? Probably quite a few. Just because it isn't immediately obvious that a person is gay, doesn't mean they aren't. Maybe Ensign Rager is a lesbian. Maybe Wesley is bi. I'd say Riker could be pansexual, given that he was once in love with a woman who came from a species with only one gender!
Lieutenant Hawk in FC was gay, was it an important story point? No, so it didn't need to be mentioned.
We don't see every part of the characters personal lives, so we don't know if Data's last sexual partner before Tasha was a guy, or if Janeway experimented with a female partner at the Academy. Going by the reasoning the "since we never saw it, it doesn't happen", that would mean that Uhura was a virgin, since we never saw her having a romance with anyone aboard ship (unless you want to count that awkward moment with Scotty in TFF). Whilst NuUhura was banging all her professors at the Academy.