It's actually 2 expressions combined, duche bag (- historically known as an instrument of cleanliness bestowed upon a dirty piece of female genitalia) and fag(Short form for faggot aka homo).
This expression is used when a duche male is sleeping with a lot of other men. He's like a male - gigolo, for men!
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi