My DD10 has friends of both sexes but during preschool and K mostly played with boys. I think she would have continued that trend except there are some unwritten rules at school that boys play with boys and girls play with girls. Her closest friends have been boys with one exception and it is still easier for her to make new friends (e.g. summer camp) with boys. I think this is because she is very athletic and, more importantly, she doesn't like to talk that much. Making friends with girls requires more talking and social skills whereas she can make friends more easily by joining the boys for a soccer or dodgeball game.
