Biology is not the only thing which makes people feel they are men, or that other people are men. Perhaps one in 100,000 men were born without a male body. We call these "transgender" or "transsexual" men. Some men can have a hormone or chromosomal difference (for example "androgen insensitivity syndrome"). Some men have other intersex conditions. Some of those intersex people who people said were female (girls) when they were born want to change this later in their lives. 