User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-4662873-20131011152546/@comment-5927468-20131014144414

I don't think it's really an issue that Nico seemed to get nervous around girls in PJO. It could be he was still uncertain about his sexuality and thought he liked girls, if only because that was the norm back in the 40s.

Also, this is hardly the first time a character has suddenly switched teams. In Buffy the Vampire Slayer, her best friend Willow comes out as gay in season 4. This happens despite the fact that Willow had a crush on her friend Xander for years and then a sexually active relationship with her boyfriend Oz. While I don't know what the initial fan reaction was, now Willow and her girlfriend are considered one of the best romances on the show.

I probably refer to that show more than I should in life (WWBD). But my point is it's not a stretch to make Nico gay.