To be fair, portraying a gay person on screen, and doing them justice, is rather difficult. Being gay isn't something you can see, which makes it a bit harder to telegraph one's orientation in a visual medium without screwing it up.
That said, if it were up to me, I would have done this: Riker, Troi, and Guinan are discussing something in Ten Forward. In the background, almost at the focus range of the camera, you see a male civilian stand up and greet another male walking up to his table, smiling, kissing him on the lips and then offering him a chair. All the while, Riker, Troi, and Guinan are still focused on the main plot or discussion, and no one comments on anything.
It would be subtle enough, without seeming overt, and would get the point across fairly simply. In that simple 10 second scene, it would be established that homosexuality is present in the Star Trek universe, that it's openly accepted, and that it's a normal type of social acceptance.