Marius working out his feelings for boys (Courf in particular) in the absence of sexual attraction was particularly difficult because of the absence of meaningful friendships in his life.
With Cosette, Cosette took his breath away so quickly, delighted and enchanted him, and Marius has rewatched his favorite Disney movies often enough that that kind of sparkly, hazy feel between a girl and a boy was unmistakable.
Realizing that the rosy glow of affection and admiration he held for Courfeyrac was more than a feeling of friendship was a little more difficult. He had no models in his head for this sort of thing. Even as his new set of friends exposed him to queer love, they’re all such an open and sex-positive group that it seemed thoroughly tied to sexuality and lust for one another. It was difficult to recognize his feelings for Courfeyrac as being related.
It’s not until les amis are spending the afternoon in a city park together and Marius watches Courfeyrac and Cosette weaving friendship bracelets for each other that he realizes he isn’t sure who he is more enamored with, and that his feelings for one are not any different than his feelings for the other.
It’s a revelation almost as unsettling as it is peaceful, but his girlfriend and soon-to-be-boyfriend help him through it just fine.