One thing I've picked up on is something about myself, and how men and women react to it differently.
So when we talk about ebb and flow, I think a lot may have to do with what's going on in bi people's lives, and what each gender can offer them. And what's missing that they can find in the other gender. I'm sure the same goes for women, during times when they can turn to other women that can better relate to them than their male counterparts, resulting in a special bond that made lead to more. Perhaps during specific times when there is a perceived need.
There are so many great posts on this of late, loving this discussion.
When i was married (to a woman) i came to think of my marriage as my "prison relationship." To clarify, it was my own beliefs and conditioning that held me 'prisoner', not my wife. i don't see my own experience as "ebb and flow" so much as discovery and flowing towards men as a result of realizing i'm 'wired' or predisposed towards men.
On the emotional/psychological side, it's the "Top" drive to create, control, penetrate, inseminate/impregnate (and a whole bunch of other descriptors) with Their Self that i have come to associate with the men i am attracted to for sexual/emotional/psychological bonding. Simply put, i see it as opposites attracting and i am wired 'bottom' and i am the opposite fit to those 'things,' and (more importantly?) to the drive/needs behind them. I.e., emotionally/psychologically my drive is malleable (be "created"), receptive/submissive to that "controlling" drive, need to be penetrated, inseminated, impregnated, etc., etc..
i know there are women who have the "Top" emotional/psychological drive, though i haven't encountered many. But i am not really attracted to them sexually and relationally beyond friendship. So, i identify as gay. But being gay doesn't mean i am sexually and relationally attracted to all men because not all men are "Tops." i may love a guy who is a bottom, but there won't be that magnetic pull between opposites that is generally associated with attraction between "opposite sex."
i've come to a place where i rarely even hook with a "versatile" guy, and wouldn't likely consider a serious ongoing intimate/sexual relationship with someone identifying as "versatile." my experience with versatile guys has been, sooner or later, they want me to 'top' them (and "top" has so many layers to me, more than just "fucking"), i am not versatile or top and i have found a lot of versatile guys have a hard time relating to that because they go both ways. i know there are versatile guys out there that go either way (i.e., way to simply put, Top or bottom), but there are guys who pretty much stay toward the Top end, and others who stay toward the bottom.
i am way oversimplifying this to make a point. i see the "ebb and flow" as more about the (complex) Top/bottom (whether male or female) disposition than gender? For me, if there were a whole bunch of women out there who wanted to "Top" me, i'd still not "flow" towards them. But from what i read, there are guys here who might be a whole lot more hetero if they could only find women who would "Top" them (again, "Top" and "bottom" are very complex to me, much more than simple sex positions). I.e., their "bisexuality" is more related to that position (disposition?) than gender. Their 'attraction' to men is more of a "prison" type relationship? More out of making do than ideal?