We tend to view sexuality or sexual preference as a very cut-and-dry, simple thing. We also tend to tie sexual preference with love, and assume that if you, for instance, prefer having sex with men, you only fall in love with men. Here's why both of these things get really tricky.
First off, having sex with someone of the same sex does not necessarily make you gay or lesbian, any more than eating a meal with no meat makes you a vegetarian. Having sex with someone of the opposite sex doesn't necessarily make you straight, either. Having sex with either someone of the opposite or same sex may simply make you confused, curious, or really into sex, depending on who you are and your particular circumstances. There are also in-betweens, as many people find themselves attracted to both men and women, regardless of their own sex, and there are many complicated combinations and absences of sex and gender that would make for an exhaustive post just in and of themselves.
Secondly, there is the issue of love, which I think is separate from the issue of sex. It seems to be clear that, no matter who you like having sex with, the question of who you will fall in love with can never be taken for granted. For instance, a person may, in terms of sexual preference, be attracted to only females (as a general rule). This person may, however, fall in love with one particular man, get married, and live monogamously. Said person may still generally only find women attractive, except for the one man she fell in love with. So, how do you categorize that? It's important to think carefully about these things, because they get all muddled up in our culture, and we can then have a really hard time defining them for ourselves.
We talk about 'making love' - and we usually mean having sex. But in reality, making love is so much more encompassing than simply sex. Sex can be one part of it, and sex certainly is intimate, but you can have passionate, intimate conversation with someone, or intense interaction in some other way such as dancing or even cooking and eating together without intending to or in actuality falling in love with them. Making love is a whole-life process, in which you devote a significant chunk of your life to focusing on a person and, as it talks about in The Little Prince, sitting a little closer to them every day. It is an intentional, directed effort - 'husbandry' of a sort. Tilling the soil so that it yields fruit. This can involve sex, and often does, just as it often involves conversation, shared activity, silliness, laughter, tears, hurt, and reconciliation.
The lesson to take away from this is, I think, that you love who you love. Sometimes it might end up being the most unlikely person, but really, that's how life is - what ends up happening is often the thing you would have least expected.
Here is the importance of all of this: we fail to understand so much about people because we generalize. We fail to understand so much about ourselves because we generalize. Because we fail to understand, we fear. Because we fear we get defensive. Because we get defensive, we aim to hurt. This can be aimed either externally or internally.
We can also use these categories and labels to try and make ourselves fit into the 'right' categories as well, so that we aren't attacked by others (or sometimes so we *are* attacked, there's always the person who needs to be a victim), so we seem to be more understandable, or so we seem to be nearer people we admire or want to be like. This doesn't do anyone any favors either, as you then miss out on developing the person you actually are, and finding people in your life who actually relate to you, rather than ones who just appear to on the surface. It may be easier in some ways, as there is perhaps less risk of injury, but there is also much less potential for real enjoyment in life.
There are so many examples of people who simply do not fit the constraints of the boxes we try to put on them (in fact, probably no single person does, unless the category was based on them, in which case one person fits), not only in terms of sexuality, but all of life. People are complicated, diverse, curious, adventurous, clever beings, and it's pretty hard to lump large numbers of them together and say "this is exactly what these people are like, every one of them the same." It is important for our own understanding, sanity, enjoyment, happiness and peace of mind to attempt to understand individual people, to have our understanding of what humanity is expanded, and our fear of the unknown quelled by knowing.