I think it has more to do with gender non conforming being lumped in with trans identity. So younger people will probably seek out their identity and as they age they will settle on wjat their identity is. Lgbt as a a concept iz starting to become outdated, because peo0le identifying as Lesbien and Gay has roughly stayed the same. Bisexual, Asexual, and Transgender has changed in their definitions over the years and have either broadened or narrowed. So the expansion probably has more to do with what Barbarian said and its more of a wider net rather than anything to do with Christianity.
Heck, Christianity has little to do with tjis since its not about a beilief in Christ, but about how one sees themselves to modern gender and sexual norms.