Yes, absolutely true re the Red Light District. Workers flaunting themselves to potential customers... very sad.
Amsterdam is very chilled and I don't think it would be an issue to go together. It purely depends on where (and when) you plan to hang out. All cities have their dangerous areas that you should avoid.
I can't really comment re crime. All cities have their crime and coming from South Africa we are taught to be very aware of these things... so I have never had any issues before. ;)