David Hayter, who penned "X-Men" and "X2" ... tells TMZ ... he loved hearing AC's take ... 'cause the dude who played the mutant Nightcrawler is 100% right ... the 2003 movie is pretty gay -- for an important reason!

Here's the deal... the film actor and Broadway star -- out since 1998 -- reflected on his diverse career, highlighting "X2" ... proudly calling it the "gayest film I've ever done, and that's me saying it" ... noting he believes the storyline is an allegory for queerness.

DH points out "X-Men" and "X2" were among the first successful superhero films … way before the MCU … and a big part of the success was relatable characters and storylines about the mutants hiding their special gifts to avoid persecution ... bigotry and exclusion being, sadly, highly relatable.

David boasts the LGBTQ+ community was represented both in the cast and crew, so the topic was on their minds while making it ... but, the writer stresses the overall message is for anyone who has ever felt hated or ostracized to be themselves and not waiver.

And, DH gave serious props to 'X-Men' star Ian McKellen – a gay rights icon – who he says was well aware of what the films could mean -- and who they might be speaking to -- beyond straight white comic book fans, so he encouraged leaning in with deeper parallels, albeit not heavy-handed.

And, of course, the “tried not being a mutant” line in "X2" was a direct reference to gay struggles, David noting the question was so devastating coming from a hero’s mother ... clearly illustrating the impossible situations people face with bigotry in their own family.