Saying this as a fag (well bi but still) it has political and sexual identity associated with it. The key is that, unlike a usual ship in most cases, people don't watch the show otherwise. They aren't into the characters for themselves or their interactions, they want to see them pair with the same gender and are fully committed to it.
It's not always the case though depending on why people watch. Straight shippers like Starco shippers are the worst in animation, and while something like Voltron is unsalvageable I've seen many actually chill gay shippers if it involves something they get invested in outside ships