Emo is actually just one branch of a whole conglomerate of darker alternative styles, but I find that people who don't know that tend to use "emo" as an umbrella term for emos, goths, punks, scene kids, steampunks, and other "alternative" styles. This really shouldn't happen. Each subculture has a general look which is vastly different from the other, with different looks within each subculture. Oftentimes, a subculture puts more importance on the music and literature it brings forth instead of the clothing, but that's another story. You want to know about the clothing, and why guys your age seem to be attracted to it.
"Emo" comes from the word "emotional," and was used to describe a genre of music. Since then, it's evolved into a whole subculture, but it seems to be most prevalent in younger teens, who usually eventually grow out of it. (What they grow into is a different story. . .) Maybe boys your age like it for the same reasons the "emo girls" are the way they are; being with them feels rebellious, a little wilder, and a bit different from the same-old. Like the emos themselves, many will grow out of it.
Or, perhaps, they like the girl for her personality instead of her appearance. That's always a good reason.