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!
Someone who jumps from one relationship immediately into another one.
Serial monogamists can not stand to be alone and often suffer from vast commitment and insecurity issues.
Because they jump into relationships immediately after the previous one has ended, serial monogamists typically don't take the time to reflect on their behavior or why their previous relationships failed; thus, they end up making the same relationship mistakes over and over again.
Person 1: Damn, Dustin already has a new girlfriend?! It's only been two weeks since he broke up with his fiance! I think he's a sociopath.