I hate that expression. As the mother of boys I hear lots of common phrases that essentially say, "Boys will do/say bad things and we shouldn't expect any better from them." I don't buy it. I do however think that boys and girls are completely, totally different creatures. Before I had kids I was all, "Gender neutral, blah, blah, blah." Then I met real children. Boys are boys and girls are girls and they are sooooo different.
I was reminded of this while dropping my guys off at school today. A boy walked by me and I overheard him bragging to a friend, "I actually know someone who ate dog poop!"