We agree that the prefix "bi" indicates exactly two. So there's that, at least.
If a person is bi, they are attracted to exactly two genders. No more, no less.
That does not mean that there can't be someone who's trisexual. Or quadrisexual. Or pansexual. The fact that some people are bi doesn't mean there are only two genders; it just means that people who are bi are only attracted to two genders.
In the OP, you appeared to argue that the existence of bisexual people is proof that there are only two genders. That's false; it isn't proof of anything.
And again, I have no comment on how many genders there are; maybe there are only two genders, maybe there are more. I frankly don't care, as it's none of my business. I'm just pointing out that the existence of people who are bi does not prove anything either way in the debate, and people on here should stop trying to make it seem like it does.