"It's what men do. A girl like you won't/wouldn't understand." There is a slight difference of meaning. With "won't" - this is definite/ categoric: I know you won't understand (because I know you and how your mind works). In the case of "wouldn't", the level of certainty is more nuanced: In my opinion, knowing you as I do, my judgement is that you won't understand. As the word "would" is associated with the conditional - it indicates that my statement is a strong belief, based on my knowledge of you. So the difference is only slight, although "won't" sounds more dismissive and direct and would be likely to annoy the other person more than "wouldn't". Either way, expect to get an angry or very irritated response! My advice would be not to use this sentence with your girlfriend/ wife under any circumstances - and if you do, make sure you are very close to the exit door ;)

Tiger, they are not interchangeable. "You won't understand" is used to talk about something in the future. In your example "You wouldn't understand" would be more correct because the statement "It's what men do" is being said at this very moment in time. If you were to change the statement to be something happening in the future, for example. "It's what men are going to do". You would then say " You won't understand".