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.
Not to be argumentative, Trek hasn't done heterosexual marriage particularly well. For the most part, character are married to the job or to the ship, and relationships are mostly passing flings. There are casual references to off-screen marriages, but otherwise there have only been three real depictions of marital life, two coming from the same series, only one being between two actual humans.
Indeed, Trek doesn't do well with adult relationships, heterosexual or homosexual or otherwise. At least with Dax-Kahn, the renewal of the relationship had to be worked out in their current bodies rather than wait for the right configuration to occur that would allow them to carry on as their marriage had been founded (and indeed, there are plenty of couples, gay and straight, who must deal with changes to one partner's body, be it due to violence, accident, or diseases--it's not that easy).