I've a friend who would be (I'm not kidding) be straight, lesbian, straight, lesbian, and, now, straight. I've brought the subject up with her, asking her why she never considered the possibility of being bisexual rather than focusing on being one or the other. She told me that she might be bisexual . . . but she doesn't care; she does what she feels. (Clearly, it wasn't a "once" for her!)
Sexuality will always be confused with the limited structure of "nature vs. nurture" debate. It's not often that people think that human sexuality is a living and evolving experience. The question is always, "You gay or straight?" Only until the past decade or so, has "bisexual" been a choice. Not once has anyone looked at me and said, "Human."
For many, sex is seen in two ways: activity vs. expression. You can express, with sex, your love/admiration/affection for another. You can have sex to explore/experiment, have fun, or have a release. Many will use a combination of both while some will freely explore the fullness of life (without social, political, religious, and inhuman restrictions).
Ultimately, it's all about what you feel inside that counts. You're human (and, dammit, God don't make no junk!). Celebrate your life, your humanity, and your soul.
In short, be yourself and don't be embarrassed to be yourself.