I headcanon that he's bi and that 1.) IRL the higher-ups were just too cowardly to make the male lords bisexual as well as the waifu (there's a lot more female/female pairings than male/male in this game) and 2.) it would be too much of a political risk for him to be openly interested in a male and he's the type to put politics before romance.

Some of his lines come off as a bit too romantic even when you're playing as male Byleth, and being bi/pan just feels like a good fit for the character. Not so much because of the superficial charm or design, but just because I feel like it fits well with his themes and dreams of equality, being an "outsider", etc. Just feels appropriate even if it's not strictly canon.