It must suck for bisexuals to be told that their sexual orientation doesn't exist. It's denigrated as a "phase," as a diversion, or as a masquerade. I was active in a gay-straight alliance in school for a while, and I heard the words "bi now, gay later" more than I cared to admit, which I thought was patently ridiculous because bisexuals are included in the LGBT umbrella for justice and fairness, and yet you had one marginalized group pumped on some stilettos and makeup and pride parades trying to bash their own allies. It was discouraging, to say the least.
For starters, I think bisexual shouldn't be used as a transition word. If you're experimenting, say you're experimenting. If you're unsure, it's okay to be unsure; say so. Be honest about where you're at if you feel it's appropriate to tell anyone. And if you don't, that's great too because it's your right, not someone else's privilege, for you to choose when and how to disclose your sexual orientation. Personally, I think it's none of my business and I don't feel I have to know unless we're in a significant relationship.
(Hell, if it's a fling, I don't care to know that anymore than I care to know what you study in school, what you want to do with your life, or the secrets you keep under your hat. Just the standard questions about being clean and if you have any extra condoms and lube. Now suck my dick already!)
I don't mean to be insensitive to Not Punny's point, but let's take gender out of the mix for a second. You got cheated and you were deceived by your partner. Your partner chose to take advantage of your feelings instead of being honest about something deeply personal that should have been established. Is there an added sting because of gender? Sure. But don't warp the fundamental wrongdoing; cheating is cheating is cheating regardless of who you do it with -- man, woman, or mechanical bull.
And I also don't get the logic behind bisexuals all of a sudden shifting sexualities just on the face of who they date or fuck either. Someone said earlier that he's not dating a guy so he's more gay. What? Did women become less attractive to you? If you were watching the same porno movie as you did pre-relationship, the woman wouldn't get you off as much? Someone needs to clarify this for me.
One of my friends in college was bisexual until she got married, and then it was as if that part of your life was written off the record like it never existed. I understand that in relationships you get constricted opportunities to do what you want to do. Those limits are necessary for a monogamous relationship to function. Sure, they can be traversed but only under the terms agreed upon in the relationship (which is why mixed orientation marriages are interesting because they, in fact, are the trial of figuring out how to accommodate everyone's interests while preserving the emotional bond of a marriage). So, why would dating a bisexual, especially if we can safely assume that their relationships fundamentally aren't any different than straight or gay ones, be any different?
Like I said, you're just dealing with a larger dating pool -- not so much an African Moon Violet. The basic assumptions of a monogamous relationship can still hold. It's merely the lack of dating someone who neatly fits into all the standard assumptions (including you should either be straight or gay, wtf is this bisexual stuff?!). And I think that genuine love and interest in a relationship doesn't mean you can't deal with those adjustments as they come.
For what it's worth, I applaud IanTheTall's relationship with his partners. Their polyamorous arrangement is certainly contemporary. But I don't think that's the key for bisexuals to be happy either. Maybe you do; maybe you don't. I don't know. But I think it's important to quit holding bisexuals to some ridiculous standard and to start treating them like regular people who deserve the same rights and privileges to love and to be loved.