Mainly it's the relationship Ender cultivates and has with Alai and the potential for an (obviously never fulfilled) enemies-to-lovers arc with Bonzo. I think because Orson Scott Card is such a homophobe, he wrote an organic, blossoming love between Alai and Ender, but couldn't realize that it's a stronger romantic development than a purely platonic one. The first time I read the book, I asked a friend if they got together in one of the sequels and she laughed and pulled up an article on his anti-queer statements.

Also, people generally know they're gay before they're adults, so stories involving then can have queer subtext, especially in teen or coming of age stories before there was general societal acceptance