He made the announcement at the end of pride month. A month where members of the LGBTQI show the rest of the world they are proud of who they are.
https://twitter.com/LilNasX/status/1145617264378548224
Apparently Lil Nas X had been giving out hints about this with his song "C7osure" off his EP "7." If you listen closely enough to the lyrics you will hear bits and pieces about his se3uality on the song. if he didn't make it more obvious, on the cover of the EP there is the colour of the rainbow which symbolises the LGBTQI community.
https://twitter.com/LilNasX/status/1145470707150860289
This comes a couple of days after a successful performance at the BET awards and "C7osure" is slowly rising to the top already having 2.4 million views on Youtube. Since making the announcement he has received a lot of support from his fans and his supporters.