TNG has Picard and Beverly, TTN has Will and Deanna, VOY has B'Elanna and Tom, ENT has T'Pol and Trip, and sure, all of those series have seen deaths of straight characters too. I just kind of wish, in 2011, Trek would be able to give us a stable gay or lesbian relationship with a major character (and I'm not advocating just switching around an existing character for the sake of diversity, but it would be nice if a gay man or a lesbian got a happy ending for once too.
Let's be fair here. Those relationships are between existing characters from canon. Characters who are MUCH less likely to be killed off in stories than any other character. And since there are no major characters from the TV shows or films who were known as gay- then it's no coincidence that those couples have not had a member die.
Then, since all of the gay relationships in the novels do NOT involve major characters from canon, they are naturally most likely have a character be killed off.
I don't really see that changing until a future movie or TV show features a major character who is gay.