The market for GLBT fiction is changing because the younger generation's experiences are different from that of their elders. When GLBT fiction was becoming more common in the latter part of the 20th century, the endings were more tragic because that was the predominant experience back then: you couldn't risk coming out and if you did, you could probably expect a life of sadness and loss. Nowadays, since attitudes are changing, the endings in GLBT fiction are leaning more towards a positive experience: when a character comes out in a novel, they can expect a fairly decent life.
The general feelings about the two gay characters in The Walking Dead might seem to be the common denominator but it's only because it's the loudest on the online forums. Many people online, I've noticed, are having a difficult time accepting the fact that times and social attitudes are changing. I have seen instances where GLBT characters were present in a show but they were shoehorned in for the sake of being Relevant! Edgy! Important! Even though they were historically accurate and entirely possible, they didn't fit into the storyline and added nothing to the plot, IMHO. However, from what I understand, the gay couple in the TV series of The Walking Dead were also in the graphic novels. While the producers of the series, again from what I understand, are taking several liberties in making the show, they may have wrestled with "do we include them as they are or do we straight-wash them?" (And if fans of the TV series "were tired of having to accept gay material on television and in movies and that WD was the only place they could get away from it", as you state above, I can't even.)
And simply because some people are pissing their pants in online forums about gay characters in a TV series, I really don't see how that can influence publishers because, as I stated above, GLBT fiction is enjoying decent sales, both in mainstream fiction and genre. Many independent publishers (Riptide and Dreamspinner, for instance) are making a killing because they focus exclusively on GLBT genre fiction, and several of the Big 5 have always produced books, both fiction and non-fiction, geared towards a GLBT audience. If they're going to listen to the angry rants of a bunch of pissed-off TV fans and begin to shy away from any GLBT books, they're really dumber than I'd expected and have no business acumen.