Star Trek: Deep Space 9 episode "Rejoined" airs 1995

Star Trek has never shied away from dealing with issues that might make some people uncomfortable.  I mean, come on!  A black woman in a non-stereotypical role?  The first interracial kiss on the TV screen? 

Star Trek: Deep Space 9 continued this streak of enlightening themes when it aired Rejoined, Episode 5, Season 4 on October 30, 1995.  Lenara Kahn (played by Susanna Thompson [Moira Queen in Arrow]) is the new host of Jadzia Dax's (Terry Farrell) former host Tobias and she arrives at the station with a group of trills.

Trills are forbidden from reassocating with former lovers/partners, but there is still a spark between them.  It might not have been the first lesbian kiss on TV or even the first woman/woman kiss, but it was a first for Star Trek: Deep Space 9. 

Star Trek: The Next Generation airs "The Host" 1991 - First appearance of the Trills

Star Trek: The Next Generation airedd "The Host", the 23rd episode of the 4th season, May 11, 1991.  It was the first appearance of the Trills, a race of symbiotic aliens, one of whom would become a major character in Star Trek: Deep Space Nine, Dax.

In TNG, Beverly Crusher is swept off her feet by Ambassador Odan (Franc Luz), who is on a diplomatic mission to settle a brewing situation between two moons of a planet.  One moon has found a way to tap into the magnetic fields of the mother planet to their immense benefit.  However, this new-found system is having a devastating effect on the second moon.  The moons are about to go to war so the home planet has brought in Ambassador Odan.

Deanna Troi senses strange things in Odan but cannot quite figure it out.  When Ambassador Odan is attacked, he is injured and the true nature of the Trill becomes apparent.  The symbiot can be saved but must be transferred to a new host immediately.  William Ryker is chosen to be the host, despite him not being a Trill.

All of  Odan's past lives are channeled into Will's memories and Odan (through Ryker) manage to resume their courtship.

The joining of Odan and Will takes its toll on Will's body but a replacement host from the Trill world arrives.  As Beverly prepares to meet the new  host, she is dissappointed to discover the new host is female.

As I mentioned before, Trills played a major part in the Deep Space Nine series with Jadzia Dax and Ezri Dax, along with appearances by many of their previous hosts.