They didn’t have to do anything (other than satisfying the studios, the advertisers and the audience — as has been repeatedly pointed out), but it would have been really nice if they did. And I don’t see why it should be wrong to voice this disappointment.
I think @BillJ sort of hits it on the head. Paramount is most interested in making money. If they thought homosexuality would have sold more shows, it would have been featured more. It may have sold more Star Trek back then, we'll never truly know in retrospect. But if they thought it would have, you would have seen it. It wouldn't have been to promote rights. It would have been to make money. Just as it is today. Perhaps a bit cynical, but true.
Perhaps we as fans put Star Trek on too high a pedestal. That it's selling a message. But the reality is probably much more basic than that. They are selling a show for profit. If a message makes them more money, you'll see more of it. If not, you won't. And if a message would lose them money, they'll avoid it like a plague (that would be for any issue BTW).
The way they use lesbianism as a shorthand for evil deviants in the Mirror Universe or the episode “Warlord” we discussed earlier is certainly problematic. A scene in “The Host” can be read as implying that they are not supposed to even exist in the future.
I'm not sure that I agree with all of that. I saw homosexuality in those instances as being more incidental. Many characters were 'evil' in the mirror universe. Straight and homosexual. I never came away feeling they were saying homosexuality itself was evil. Bashir was a bad guy in the mirror universe and he was straight. Ditto for Sisko. They may have been guilty of overplaying the lesbian part for the benefit of horny guys watching it. But I'm not sold that they were saying homosexuality was a reason the mirror universe was bad.
I certainly didn't feel that way about "Warlord." The character was a bad guy (now girl). While in Kes I believed he still thought of himself as a guy so it probably never even registered that there was anything homosexual going on. Kes was just a vessel, a useful vessel for his desire to conquer. And what's the issue with "The Host?" I think they barely touched on homosexuality there. Beverly had trouble adjusting to all the changes in Odan. And when Odan was now a woman that was a bridge too far for her....I always believed partly because she is a heterosexual. Partly also because of the frequent host changes. Her comment about being more open to the changes I interpreted to mean the host changes, the first host, Riker, now a woman. That would be a lot for anybody to take. Physical attraction is still an important aspect for most people, not the only one, but a factor yes. And there's nothing wrong with Beverly not being attracted to other females in a romantic/sexual way. Any more than a homosexual would start a relationship with someone of the opposite sex, a heterosexual is not likely to be in a romantic relationship with someone of the same sex. So I think it's perfectly natural for her not to want to continue a romantic relationship with Odan.