My girlfriend said that she was bisexual when we first got together. (I am lesbian.) For the first couple of years we were together, this was no problem, since she had never had sex with a man, and her bisexuality was more theoretical or abstract.
The first time she told me she wanted to actually experience sex with a man, I was distraught. We had a big fight about it, and she decided it wasn't worth the trouble to pursue it. But it came up again a year and a half later, and this time, she was insistent that her life would be incomplete if she did not get to experience something she was so curious about. I reluctantly agreed. I didn't want to break up with her, and she sounded willing to end the relationship if I couldn't fully accept who she is.
She went ahead and did her experimenting, and I can't honestly say that it had a negative impact on our relationship—from her side of things. She seems as loving and kind as ever, and continues to desire sex with me on a regular basis. But I can't seem to let it go. I feel as if I resent her bisexuality. I wish this had never happened. My resentment is coming between us, and I don't know how to let it go.
I'm not sure why you objected to your partner having sex with a man. But I can see two main possibilities. One is that you feel competitive or insecure regarding men. The second is that you are a monogamous person who doesn't want her girlfriend having sex with anybody else, regardless of gender. Let's look at each of these.
Regarding the first issue, you certainly have some good reasons to feel negative about male/female sex. The larger culture repeatedly sends lesbians the message that sex between two women is somehow inferior or incomplete. Just the scarcity of images of lesbians in the mass media is enough to create this message. Any lesbian who has had someone leave her to be with a man knows how much shame and agony is involved in that experience. It seems to confirm every antigay bit of bigotry.
Of course, the opposite is also true—many straight men are terrified that a woman will automatically understand what another woman needs sexually, and give her unimaginable pleasure. I think this is at the root of a lot of our society's suppression and persecution of lesbians. Many men are afraid that if the playing field was level, they could never compete with "feminine intuition" and the rapport that they imagine exists between two similar bodies. Women also sometimes leave men so they can be with other women. And men who go through that are as shamed and torn up as their lesbian counterparts.
As all lesbians know, you can't count on the presence of two xx chromosomes to make the sex hot. Women are complicated beings, and what pleases one will not necessarily tickle another's fancy. The fact that there are two women in bed doesn't remove the need for communication and experimentation to achieve the best possible level of compatibility and satisfaction. Sexual orientation also plays a role. If a woman is only attracted to men, the hottest lesbian sex in the world will not turn her head.
I can understand your feeling upset that there was a crisis in your relationship that forced you to face the fact that you and your lover are two different people, with different sexual needs. Imagine that you, as a lesbian, somehow got trapped into marriage at a young age, and didn't face your need to be with other women until you had been with your husband for several years. What would you do? Would you be able to ignore your sexual orientation and just keep on being with him? Bisexuality is every bit as real as homosexuality or heterosexuality. Your lover needed to test her self-perception against reality.
The fact that bisexual people are capable of relating to men or women has given rise to the myth that they are not capable of being monogamous. I don't think this is true. For one thing, being bisexual doesn't necessarily mean that you have a 50/50 approach to both sexes. Some bisexuals desire one sex more often than the other; or they may be capable of bonding emotionally with one sex, but not the other. (Or they can be 50/50.) Bisexuals also vary in terms of the types of relationships they want. Some want the freedom to explore passion with more than one partner—others feel that if they make a commitment to someone, that also entails monogamy.
I know that the monogamous ideal is 100% fidelity. Supposedly, this means that your partner makes you a priority and will stay and work on problems rather than just leaving. It means you can expect emotional support and mutual desire. But in the real world, I have rarely encountered a relationship that was 100% monogamous, especially if it lasts longer than four or five years. People change. New sexual fantasies or needs emerge. They get curious. They may be temporarily alienated from one another and have an affair, or just make a mistake. Being compassionate toward these shortcomings and able to forgive them is a major challenge, but I think it is worth learning how to do this, if only for personal growth, and to keep a woman you love. If someone in a full-fledged, polyamorous relationship can be there for their primary partner, surely this woman can be there for you.
I suggest that you do some journaling about why you are having trouble letting this go. Can you hear, on a deep level, that your partner did this to understand herself better, not to hurt you? Can you allow her intentions to change how you feel about the meaning of what she did? Try to focus on the here-and-now rather than the past. If she is with you, right now, focus on enjoying and celebrating that rather than dragging old anger into the room. Visualize your anger as something toxic that you have to release before it poisons you. Remember that you could have been with another lesbian.
You chose to be with a woman who said she was bisexual. She did not deceive you about her nature. Take responsibility for that choice, and resolve to love her as she is, for her truth, rather than expecting her to conform to your ideas about who she should be. She didn't go behind your back and have an affair. She talked about what she wanted to do and got your consent. You decided then that it was worth it to keep her in your life. Has that changed? If not, don't allow negativity to drive a wedge between the two of you. If you do that, you might as well have broken up with her before she had sex with a man.
If your lover was in a car accident and died tomorrow, how would you feel about the current state of the relationship? What regrets would you have? I've unexpectedly lost people in my life who I thought I could count on to be here for a long, long time. Maybe you have, too. Death teaches us a powerful lesson about having bigger hearts and staying emotionally current with the important people in our lives. You know how much you would miss her. Allow that feeling to control the way you behave in your daily life.
Give yourself time, and be gentle with yourself. This will hurt less as you continue to have the experience of being in a healthy, close relationship.