If it's not relevant to the story... what's the point? Or do you just want a token there for the sake of being there?
What's the point of all the heterosexuality on various shows? Was it relevant to the story, or just there for the sake of being there?
There have been numerous one-shot (and sometimes longer) romances on all the shows. Why couldn't some of them been between people of the same gender? Was the heterosexuality of those relationships "relevant to the story"? Only in cases that involved reproduction and children (e.g. Worf and K'Ehleyr, Trip and T'Pol). Most of Trek romances didn't, therefore the heterosexuality wasn't relevant to the story. So why must homosexuality be?
How do you know we didn't? Unless they came onto the bridge and announced "I'm gay" we never would know.
Erm, by seeing them in (or at least having them refer to) a relationship with a person of the same gender? I don't remember anyone in Trek coming into the bridge and announcing "I'm straight" either.
You want to know how you do it?
Caprica, for instance, has a character who who never announced "I am gay" and nobody on the show called him "gay" or "homosexual" (for all we know, those terms might not even exist in their society), but he is married to a man, and casually refers to hitting on guys in his youth while his brother was hitting on their sisters. And nobody on the show bats an eyelid about it, it's treated as completely normal and everyday thing. That's enough to convey that their society has a different attitude towards homosexuality than ours does. It's not even the defining characteristic of the character, it doesn't affect his "storylines", he is not "the gay guy", he is Joseph Adama's macho gangster brother, and his sexual orientation is just another feature of the character that is not an issue in itself.