While Voltron certainly stepped in it with Adam, there are a few factors that complicate the entire situation. For starters, there was the general assumption that the two would meet, when neither Montgomery or fellow showrunner Joaquim dos Santos actually said so. It was said that Adam was a part of Shiro's life, but never once did anyone in a position to make the claim say the couple's story would pick back up where they left on Earth years ago. The two had effectively broken up once Shiro accepted the Kerberos mission, and Adam was still important to Shiro personally because he was The One that Got Away. He's just a background character who informs Shiro's past, in the same general way that Allura's father or Lance's family informs theirs.
There's also another part of this situation that has to be acknowledged; namely, that the Voltron fandom has historically been incredibly messy. Since its initial premiere, the show's been caught up in ship wars that are more toxic than normal, thanks in large part to the Paladins' ages being up in the air until fairly recently. While it was clear early on that Shiro was the oldest of the five, the showrunners haven't been willing to give exact ages, and the wiggle room opened the door for some fans to ship him with the younger Paladins, Keith most of all thanks to it being clear the two knew each other pretty well before the show's start. The discourse about character ages has long been an issue with this fandom, with many declaring anyone shipping Shiro with a younger Paladin is a pedophile. It's become so ugly, even people who aren't fans know about it, and bring it up as examples of fandom gone bad.
It should also be acknowledged even had Adam lived, there's a really good chance that he would never have been given a fair shot by the show's fandom. With Shiro being a beloved character who's been through so much, Adam would've come off as a jerk at best or abusive at worst for what is actually a fairly human reaction coupled with the fact that Shiro put his health at risk pre-Kerberos due to having a degenerative muscle disease, and it causing them grief in the past. That's not a great start for a character, and the show being so dominated in ships as it is means Adam was likely going to be generally viewed as a stepping stone to Shiro/Keith, Shiro/Lance, or whoever a fan wished to ship Shiro with.
There's also the corporate side of things, specifically that Dreamworks has been fairly cagey with Shiro more than most characters. He's the leader of the Paladins, and serves as the "face" of Voltron in many respects. Originally, Shiro was supposed to die following the fateful battle with Zarkon at the end of Season 2, but Dreamworks decided he needed to come back, leading to clone Shiro.Fursther, iIf the studio had its way, Shiro wasn't going to be gay at all, and lines were admittedly recorded that just had Adam and Shiro as platonic roommates. It's possible even with the pushing the cast and crew did, the company's final say wasn't one they all agreed on.
Does this make Adam's death better? No, and the reactions of disappointment and frustration are valid; it's genuinely the only bad spot in an otherwise spectacular season. However, it's incredibly important to note that it's not a show being intentionally insidious, but the result of the best intentions being paired up with a studio that's still afraid to let people of the same gender kiss. And it's a shame, given how great of a character Shiro is, and how his moment was soured by the universe yet again.