You know, I just clicked around their site, and I'm genuinely unsure. All of the YA titles that I checked out seemed to be straight, but the description of "What is Clean" is so vague, probably because they don't take unsolicited submissions and don't need to post clear guidelines. I know "clean" usually means no/light swearing and no sex, but it's really hard to tell where they stand on queer content.
I'm actually kind of curious now, if anyone else knows.