Questions for lesbians and Bisexuals:?

OK, I am honestly asking this in the most respectful way possible. I'm gay, and have been all my life. I could never see myself being attracted to a woman. (No offense..) But basically, a lot of my friends "have" been lesbian or bisexual. By have been, I mean that they turned straight completely. Mainly bisexuals, though. And I hear about a lot of people saying that lesbians are only lesbian because they had a bad experience with a guy? Whether it be rape, or just a bad relationship. Please clarify this for me, because I truly want to know if lesbians and bisexuals are just putting on a front. I'm not trying to offend anyone btw, or start something, because I hate when people ask me if being gay is a choice, but I've never met anyone gay who turned straight.