I didn't really know where to put this, because I am not personally having this issue. It's just a question that popped into my head. If you look at the statistics the number of people that are actually 100% straight or 100% gay, are much much lower than the number of people who identify as gay or straight. Like for me, I do identify as gay but from time to time I will randomly think a girl is attractive and it will be gone again shortly. I would say I am about 97% attracted to guys, and about 3% attracted to girls, but I think the percentage of the time that I am attracted to girls is so minuscule that I am comfortable identifying as gay. If you do identify as gay or lesbian, do you ever have feelings of attraction for people of the opposite sex? What do you consider to be the "dividing line" between being bisexual and homosexual, or do you think it is more up to the individual person than anything?
In my opinion, the dividing line is your answer to the question "can I see myself in a long-term relationship with someone of a gender other than my own?" If the answer to that is "yes," you get to call yourself bisexual (or whatever label for your multisexuality you want). If you answer "no," then that's what I would call gay.
Definitely an individual decision. Sexuality is so complicated, as it is so much more than the name implies! What if you're sexually attracted to women, but fall in love with men? What if you're part of the MSM community, and you're having sex with dudes but still consider yourself straight? People should be able to define their own orientation, however they see it. Though there are always going to be people who try to police the thoughts of others. If, as a woman, I'm sexually attracted to men and women, but only fall in love with women, am I bisexual or a lesbian? I can ask for advice, but ultimately that is up to me to decide, and no one else. I agree with Pret Allez that the serious relationship question is a good one. Though I think the question right after that ought to be, "Exactly how important is sexual chemistry to you in a long-term relationship?"