I hope this sexist stereotyping is just a bit of fun:-)
Men don't remember birthdays/anniversaries because they have more important things to remember like the day of the FA cup, the Grand National, Twickenham, the rugby league cup, when they're going on that fishing trip... Why is it that women can't remember these dates and always seem to hit them when they are organising family events or outings.
It is a survival technique: women don't like birthdays/anniversaries, it reminds then that they're a year older, or for how long they've been tied to the kitchen sink.
In December I have to remember my wife's birthday, our anniversary, and Christmas. Now, Christmas is easy, it has been flung at us left, right, and centre since late July; birthday and anniversary are not so easy, they fall within a couple of days of each other and have to be remember the right way around or I'm in trouble.