As author Chimamanda Adichie cautious in her famous TED Talk, we must beware of the danger of a single story. We must reflect on the "single stories" of others we ourselves carry, and we must take steps to create, and promote, other narratives, especially of marginalized people and experiences.
So we absolutely need more stories about same sex partners (any kind of queer partners, actually) that don't end in tragic death. LGBTQ youth need to see stories about people like them, which have happy endings (or at least where everyone's not dead!!).
No really, we need happy endings too.
But that is not the same thing as saying that there is no place for queer stories which do end in the death of one or both partners.
I have a hunch that perhaps we see so many of these stories today because of the legacy of the HIV/AIDS epidemic, and the many personal and community traumas endured by the gay community on account of that. Perhaps many straight people grew up only hearing about gay people in this context, and so in their minds, the "single story" can only end with tragedy and death. Or perhaps they've lost friends or loved ones due to AIDS themselves. Perhaps also for many gay people, writing stories which end in tragic death is a conscious or unconscious way of processing this trauma, even part of the psychic healing process -- a way to deal with these personal and communal wounds.
On top of this I would like to add that not all LGBTQ people can safely come out, so please be cautious before judging people who write such stories. If you don't know them closely and personally, you don't know what they have endured and survived.
tl;dr: We need more diversity of narratives, so we can create MORE SPACE for narratives, not shut out narratives that wrestle with dark and uncomfortable subjects.