So according to dating websites there's no such thing as bisexuality!?

Now I've never suffered discrimination due to my sexuality (I'm bi) except for when I was first outed at school a few weeks before we left for good and a girl came up to me and said, in that attitude full voice teenagers normally save for their parents, 'Ugh, are you bi?' and I simply shrugged and said 'yeah' and she slunk away because it's hard to bully someone who doesn't care (if only I'd realise that 5 years earlier, sigh). I think the reason(s) I've never had a problem is because I'm bi rather then gay and that's generally more acceptable to close minded smeg heads (and men who thinks that means they'll get a threesome), I'm fat and a geek so there's more obvious things to bully me about and because as I already said I'm fat, and I'm strong to boot, so I'm quite forbidding. However something came to my attention recently that really pissed me off and for the first time I felt discriminated against (because of my sexuality, as I said I'm fat and a geek and whole plethora of other things I'm regularly discriminated against for).

A friend (and no that's the 'friend' as in 'me but too embarrassed to say', I did try online dating once, it was crap) tried to sign up to a dating sight that shall remain nameless (cough *match.com* cough) today but left feeling annoyed, frustrated and discriminated against after trying to answer the very first question. It was not, as one might expect, their name but what they were looking for and they were presented with the following options: 'I am a man seeking a woman', 'I am a woman seeking a man', 'I am a man seeking a man' or 'I am a woman seeking a woman'. Now, I'm seeing a major flaw in those options. What if, like both me and her, you are a woman/man seeking a woman or a man? Some of us like to keep our options open but according to match.com we can be either straight or gay and not bisexual. Now I find this quite offensive and like she was would be very annoyed by this, I don't want to choose which one I have before I even start looking. A similar situation arises with one of my friends who whenever we talk about love/sex/whatever she asks 'Which way are you swinging at the moment then?' which I always find not offensive exactly but a bit baffling. Just because at one time or another I am dating/shagging/whatever a man or a woman doesn't mean the other sex is excluded from my sexuality. I mean if a straight/gay person is going out with someone does that mean all other people of their chosen gender doesn't exist anymore? Yeah OK they're not doing anything about it but nobody can claim not to notice other people when they're in a relationship. On a side note, personally I see nothing wrong with looking so long as you don't do anything about it. I'm not even that fussed if they fantasise about them, I mean if they need to fantasise about you then you're doing summat wrong, you know? But that's just me and is besides the point. It doesn't really matter who I'm seeing at any particular time I'm still bi. And I would like to know that if I ever tried to again there would be a space for me on internet dating sites and they wouldn't try to restrict me.