To reply you question "does being bi (or queer or pan or however you describe yourself) work in phases like that ie: totally one thing and then totally another?", I would say it really depends on the individual.

I'm bi, and I believe sexuality is fluid. For me, I could be very interested in girls for half a year (like 90% interest in girls, 10% on guys), while the other half I could totally fall crazy for a guy that I'm eyeing (so in terms of percentage, 95% guys and maybe 5% for girls).

I really have no answer for why "suddenly" your friends become hetero after all those years being interested in women only... other than the fact (or my belief/experience) that sexuality is fluid. (I'm still pretty new to my bisexuality, so I can't really provide any further analysis. sorry) But hope the above helps!

My experience is not at all like your friends', but I'd never suggest this means they lie or are insincere or anything else. I always try to give people the benefit of the doubt about their own sexual interests.

I'll admit, I have had an ongoing joke with a couple of friends for a couple years about my having particularly gay and particularly hetero days. Every once in awhile it would occur to me or to another of my friends that I seemed to be having a disproportionate number of thoughts or making a disproportionate number of comments about intense sexual attraction to one sex or the other. Often, I'd make the umpteenth comment about how pretty a woman was and someone might say, "Wow, you're such a lesbian today." I'm pretty sure it's really not that I'm having an "especially gay day" or a "super straight day," not a matter of swinging back and forth, but really just about the coincidence every once in awhile of coming across many people of one sex I'm attracted to and no particularly attractive members of the other sex for a day.

But I guess the question for me would be, even if I went several years of only being interested in dating people of one sex or the other, like years of "really gay" or "really hetero days," would I quit calling myself bisexual and just go with what fit for the current decade? I'm not sure...I mean, knowing I had absolutely been romantically and sexually interested in both men and women at some point in the past, I think I'd stick with bisexual, even if I happened to have an "especially gay" or "super straight" decade. It just feels like my identity at this point...but who knows, maybe that would change too if my attractions changed.

I'm gonna have to jump on the bandwagon.

I think it depends on the person. I'm another one of the "labels suck" brats. Not because I'm trying to be PC but because I find them to be too inaccurate for my taste.

There's a thread here that talks about Klein's Sexual orientation grid. It says that a person could change their orientation during their lifetime. The only studies I've seen that track women's sexuality over a period time came to the conclusion that it can change over the years.

well, being bisexual myself, i find my experiences to be not evenly hashed-out. i don't equally like both a man and a woman at the same time, or one right after the other consecutively in any alternating pattern. for me it does go through phases, sometimes. the extremity depends. a few years ago i was still coming to terms with liking girls, and had a massive longtime attraction to a man, and thus i felt mostly straight. in the past few months i've felt extremely gay, possibly because i had a short thing with a girl a few months back, plus the l word was on in march, plus... well, whatever else. i don't fully understand it myself. i don't even know if those are truly influences or whether i create them myself.
so yes, phases and cycles can occur. that may not be it for everyone, but it seems to be so for me.

however, what your friends are doing is rather extreme. being totally involved with the lgbt scene and then abandoning it when they get boyfriends? that's really pretty fucked up. maybe they thought they were gay, and now they think they are straight? maybe they really were going through a bi-curious phase?
your friends might view sexuality as an all-or-nothing thing: either you're gay or you're straight. they probably don't view themselves as bisexual.

the thing is, human sexuality is variable throughout life. it's not a constant thing. your friends may very well be bisexual, but think that since they're only attracted to one thing at one time that they've "switched". they're not thinking about all their attractions over the span of a lifetime.

tl;dr: yes, bisexuality can mean phases and mood swings, for some. but your friends don't appear to acknowledge their bisexuality (if bisexual is what they are) and have acted rather brashly severing all ties with the lgbt community.