I don't think there's anything that states explicitly that they are only straight? Gale has an obsession with a god, and Wyll has a past with a female cambion, but there's nothing locking them to a sexuality?
Gale and Wyll only (so far) flirt with female NPCs and will only (so far) sleep with Lae'zel at the party, not with each other or Astarion. They're very straight-coded outside of interactions with the PC. I'm not sure if Larian realized they were doing that.
Wyll doesn't even show interest in the PC until the party. And getting Gale to show interest - i.e. triggering his pre-party cutscenes - is stupidly difficult at the moment.