i have to laugh at this just due to my experiences.
i've dated and had sex with gay men, most of which have never been with or dated a woman before.
i have also had MANY of my ex-lovers decide that they were formerly straight and head down the gay or bisexual path. some have come back to heterosexuality after experimentation, some have stayed solely with men. today i just found out TODAY (thanks, myspace!) that the guy i dated during high school and my first year of college is now openly gay. i credit this to my hanging out with a very diverse crowd of individuals who were openly homo- and bisexual. i was put off at first if an ex-bf decided he would date men, but as i got older i realised it was never about ME. their lives and their preferences are their own.
my experiences with having straight men date men after they were with me OR gay men wanting to have sex with me has earned me the nickname with my friends as "the spatula." yes, we all know that a person cannot "flip" another person's sexual preference, but my friends all look on with amusement.
my first gay-to-straight experience was during my undergrad years with my best 'girlfriend,' jon. jon and i had created a 'girls night in' with a VHS tape of "all that jazz," some popcorn, and some talk about the cute boys that were in our classes. as we're propped up on pillows watching roy schieder prance across the stage, i felt jon's hand move from my knee up to my nether regions. i spun around and glared at him, and he put both of his hands over his eyes and SCREAMED at the top of his lungs, "OH MY GOD, I HAVE NO IDEA WHY I DID THAT BUT I WANT TO HAVE SEX WITH YOU BUT I'VE NEVER SEEN A WOMAN'S VAGINA BEFORE IN MY LIFE WHAT IS WRONG WITH MEEEEEEEEEEEE!!!" ... and ran out of my apartment. we began speaking again about a month later, but things were never the same. it made me sad because he was my best girlfriend.
so began a series of back-and-forth 'flipping,' as my friends like to call it. i don't think that we as mammals are 100% straight or gay as we can be influenced by going with what 'feels good.' falling in love is the same way.
to answer marluc's question, even after all of my straight-to-gay and gay-to-straight experiences (and in between!) i would not date an openly bisexual guy. i'm not sure why... i think it is more of a trust issue rather than anything else. i know that biologically, it's easy for men to get together and get off (usually, i've gotten off in less than 5 seconds before quite easily!) so i might always wonder if my bisexual bf has found someone male that he likes to sex up better than me. but it's a totally different experience to be with either sex, so i don't know why i feel that way. ????