That's because he's a go***amn monk! That how he was taught to act! Monks don't discriminate.
Wan himself is more Ace than anyone else!
And no, Each avatar has it's own spirit plus Raava, So raava basically just rolls the dice on everyone. And technically yes, Sexuality is part of the spirit.
And even if Aang was lgbt, They would've included him in the "LGBTQ+ Characters" Category.