It's important to remember that sex is not the same thing as gender.
Sex is biological and physiological, and is the same across time and culture. Males tend to be taller and hairier with more upper body strength, while females tend to be shorter and curvier with wider hips. As has been stated, almost everyone you will ever meet is either male or female, and the exceptions are rarely even noticeable.
Gender is a function of culture, specifically what that society expects of the masculine and feminine. In our society, men are more likely to be called Steve, wear suits, and work as mechanics or soldiers, while women are more likely to be called Stephanie, wear dresses, and work as nurses or interior designers. These do change over time and across cultures, and it isn't too hard to think of gender expectations that have changes even within our own lifetimes, such as long hair on men.
Most boys and men in our society follow masculine expectations, of course, but many may prefer things that society considers feminine; maybe a guy likes figure skating, or scented candles, or romantic comedies, and few would think twice about it. Same goes with girls and women who might watch the NFL, or collect Hot Wheels, or watch a lot of James Bond, no big deal. I'm pretty sure we all know a number of people who like things that most of their sex don't.
Some might like more than one of those, and even fewer might like a lot. Some — a minority to be sure, but still a noticeable amount — might prefer most of what society expects of the gender opposite to their own sex. They're literally trans (Latin for "on the other side of") gender.
Trans is not determined by sex or chromosomes, even for those who have had surgery. It also has nothing to do with sexual preference; there are trans people on every point on the gay-bi-pan-straight-ace spectrum. Being trans is just about how someone chooses to live, and how they live up to the expectations of the culture.
That's all; carry on.
