I don't recall hearing the interview where they said they were only doing it for money and girls, but I assume you have heard them say that, or you wouldn't make such generalised statements, hmmm.
Anyway, even if they are doing it just for money and girls, why shouldn't they? Plenty of people do their jobs just for money, so wouldn't you do a job if you got both mnoney, and girls?
Seriously though, there are guys who listen to nsync, and I would find it hard to believe that they are not doing it for a love of music, as well as any other reasons they have.