Songs And Lyrics
Silly Love Songs and Intercourse with You have been in music forever, and both became a huge part of Glam Rock and Hair Metal. That said, a very large portion of both, in those genres, are gendered or heterosexual. X Japan was one of the first bands that, while the sexuality of its own members is quite up for debate, that managed to break the barrier and the expectations - none of their Power Ballads / Silly Love Songs are explicitly gendered (e.g. the singer in "Forever Love" or "Say Anything," or "Crucify My Love," for example, could be singing to or about a woman, or to or about a man...) and at least one of their Intercourse with You songs is explicitly about male/male sex and the desire and pleasure of it ("Stab Me In The Back") while "Standing Sex" could be interpreted to refer to either gender. Even "White Poem" could be seen as a non-gendered song, though its PV and live are femdom BDSM (which is, itself, pretty revolutionary re gender roles in Japan at the time). The Visual Kei gender-bending itself (which the band and Yoshiki and hide pioneered) was itself a direct challenge to Japanese and Western cultural gender roles and mores alike.
X Japan was also one of the very first bands to do man on man fanservice onstage in Japan as a matter of course, sometimes with displays that could be seen as incredibly, over-the-top Camp Gay. (If you don't believe this, look up any given one of their covers of T-Rex's "20th Century Boy" and keep your eyes on Toshi and Taiji as they make the song even more over the top than it was in its original Glam Rock incarnation.)
Offstage, Yoshiki and Toshi and hide were some of the most outspoken HIV/AIDS and safe-sex activists in a country that, at the time, wanted to deny that HIV/AIDS even existed within its borders (and actively did deny this all the way into the late 1990s).
And this was done by a band in the late 1980s and early 1990s in Japan. So as well as pioneering Visual Kei, it could also be argued that they were one of the first (and still one of the rare) bands encompassed under the hard rock/heavy metal umbrella to also be overtly at least allies, if not themselves gay or bisexual or offstage gender-role-variant, and to have the courage to carry that onstage and celebrate their ambiguity both onstage and offstage.