I am a well-adjusted late twenties male with a steady girlfriend. Lately, I have been increasingly fantasizing about various types of anal penetration. I have discovered that during masturbation, this brings me very intense pain/pleasure. Even though I think it may be satisfying to experiment with a vibrator or other objects inserted by my girlfriend, I find myself drawn toward trying the real thing (i.e., having anal sex with a penis). The problem is, I definitely do not consider myself homosexual and, in fact, the thought of the actual act with another man actually repulses me. Am I trying to have the real thing without going through a bisexual experiment or what?
One beauty of human sexuality is its freedom of expression through a range of pleasure possibilities. Fantasies and masturbation are safe ways to "try out" various activities that one may or may not feel comfortable trying in reality. When exploring fantasies with another, there are many ways a person can give and receive pleasure through touching, holding, kissing, and intercourse. Sex can involve just one or two areas of the body, or everything could be turned on, from head to toe. People can be themselves or play roles. They can use fingers and mouths, perhaps picking up a few sex toys along the way. In a healthy, consensual sex encounter, one can communicate passions, or can "take the Fifth."
This "sexual constitution" can be compromised when a person feels pressured to hang labels on their thoughts and actions. Sometimes, labels can be empowering and sometimes, they act like a ball and chain around people's desires. Your enjoyment of anal penetration, from whatever source, and for whatever reason, does not in and of itself mean that you are bi-, hetero-, or homosexual. Throughout our lives, we all live on a dynamic continuum of sexual desires. It is often socially and culturally influenced discomfort with sexuality in general — let alone private behaviors not defined as "normal" — that push us to pigeonhole our sexual thoughts and actions into the comfortable and manageable boxes of gay, bi, and straight.
Kudos for being in touch with whatever flies your flag. It may be that fantasizing about receiving anal sex is as close as you'll want to come to having sex with a man, or it may be that one day, you'll be interested enough to try it. Have you shared your fantasy with your girlfriend? Have you mentioned your interest in experimenting with toys? This may be a way to enhance the fantasy, if you let her in on it. You may decide to pursue anal sex now, later, or never. Sexual experimentation with men does not mean you have to identify as bisexual or gay. Sexual experimentation of any kind does not mean a lifelong commitment to a given behavior or identity. It's more like a life-long adventure.