You probably have been asked this before, but bear with me, please. How do you really KNOW where you are on the sexuality continuum? I am a female who was raised by a gay uncle and, not surprisingly, I have always been attracted to metrosexuals. I am emotionally attached to men, but since I was a teen, I loved to look at women. But always in magazines, television, or porn. Never in real life. But I always attributed it to the lure of the forbidden. Now I have had two close encounters with women that I am kicking myself did not come to fruition. I find myself getting more and more horny courtesy of the ladies these days. I don't see myself as wanting a relationship with a woman, though. So where does that put me? Hopefully in a compromising position!
I should explain that Alfred Kinsey was an American sex researcher who compiled groundbreaking research on male and female sexual behavior in the 1950s and early 1960s. His team created a scale to describe sexual orientation, instead of the simple distinction of straight versus gay, because they were encountering complex data about how sexual desire and behavior changes as people age. The scale goes from 1, which represents someone with exclusively heterosexual fantasies and behavior, to a 6, which represents someone with exclusively homosexual fantasies and behavior. The 3 would have equal fantasies and experiences with men and women. But there are many other possibilities.
We are taught that sexual orientation is a fixed and inherent part of a person's identity. So you only need to figure out your identity once. Then you're set for life. You can start telling others how you identify and plan your life based on that knowledge. But the truth is not so simple. Devoted heterosexuals can fall for a very special member of their own sex, and vice versa. Bisexuals can develop a preference for men or women, and decide that being heterosexual or gay is a better fit. What works when you are twenty might not be what makes you happy when you are forty. People's beliefs about what is right or wrong, the presence or absence of any given erotic option, prejudice against certain identities or behaviors, family loyalty, a monogamous relationship—all of these things can limit, shape, or focus our sexual behavior.I personally think that what we really need to explain is why everybody isn't seeking out every possible source of sexual pleasure. But then again, I'm a perverse slut who's dedicated a lifetime to checking out all the nooks and crannies of human sexuality.
Back to you, then. You grew up with a gay uncle, so you had a window into a world that many straight people never see close up. To the extent that he was a role model for you, you tend to feel comfortable with and attracted to men who are nonhomophobic and devote a certain amount of time to their personal appearances. You want your testosterone in a metrosexual package.
And yet there are these fantasies that don't quite fit into a heterosexual box. (Which you don't fit already, for many other reasons.) You know you want men and can enjoy emotional closeness with them. So why the persistent thoughts about women? As you say, the forbidden has a certain fascination. But if that was all it was, you would probably be happy to keep your lesbian porn and masturbation fantasies; you wouldn't be kicking yourself for having a chance to play around with another girl, then getting that opportunity yanked away.
Sometimes we know our sexual preferences because they just announce themselves very clearly to us. We only want one thing. It's highlighted in big bold letters. And sometimes we can only know if we go find out. I don't see any way for you to know if you like having sex with women or not, unless you experience it. So whatever it was that you did or said to sabotage those other opportunities—cut it out. Say yes instead of maybe or no. It's okay to experiment as long as you are honest with the other person. Don't say, "I'm in love with you" or "I know I'm a hardcore dyke" if all you want is to see her naked body and touch her and see how it feels to have her touch you. (Or perhaps you merely want to begin with a kiss. If that doesn't suck, you can go to second base.)
In a homophobic world, all it takes is one "gay" experience to brand you for life. But this is simply not realistic. Lots of straight men experimented with same-sex activity in their teens. A lesbian who has had one good experience with a man isn't a closeted heterosexual or even bisexual, unless she happens to feel that way. I wish it was easier for those of us who feel some level of same-sex attraction to check it out without being judged so harshly by others. Some of that judgment (just to warn you) can come from lesbians who don't want their hearts broken by women who will ultimately choose men over a female lover. But you are not the only woman in the world who wants casual sex with no strings attached. I hope Sappho sends a lovely stranger your way sooner rather than later.