Coca Cola definitely tried to market Coke Zero to a male audience (it was described as 'Bloke Coke' in UK newspapers [1] around launch), but I think it's popular today not because of the marketing, but because it's a good product. To me, Diet Coke tastes artificial and... bland. Coke Zero tastes similar to regular Coca Cola, but has a distinct enough character to be appealing in its own right. In my experience, this is what most Coke Zero drinkers will say.

[1]: http://www.independent.co.uk/news/media/introducing-bloke-co...