Biphobia as a content warning is a bit subjective, but as a bi person I included it because

Alexander is portrayed as gay for most of the book, but when asked what he is, he basically confirms he's bi or pan without using the word. Oliver also reads as bi or pan eventually because he admits to having loved James romantically while also clearly loving and being attracted to Meredith, and uses the whole 'my love transends gender' thing as his reasoning.

I chalk this up to time period accuracy, being that in the 90s societal attitudes were different about things like bisexuality, but I know it might still bother some people, hence the warning :)