You can't judge all relationships that have some level of nagging the same.
Why do we try to make blanket statements like this? It makes absolutely NO sense.
I have seen relationships where nagging is because the person is naggy, where they want way more than another person can offer, they have different priorities, where one values certain things, so nags the other, and the other doesn't put those things even on the priority list. I have also seen instances where someone nags because they are not getting what they expected from the relationship, and don't feel like they have a partner.
Across the board, it usually speaks to some deeper issue that could be the fault of either party, or no one's fault at all, just a factor of unfortunate circumstances.
The thing is guys that women actually statistically whine and bitch more than men do.
This usually in relationship is perceivable in any fights and argument. I don't know why but women just have the inborn ability to annoy people... They even do it to each other and they never apologize to anyone unless they are forced to.