Yes, and No. There’s more to it than you might think; As with most things, it is not so black and white.
By nature and nature being the keyword here, all humans that are operating within healthy parameters are merely sexual. It’s one of the primal urges: It’s part of the id. The pleasure center cares only for pleasure and cares, not how it goes about getting it. It’s the part of the mind that gets aroused even when it sees two other animals get it on.
But then we have the ego and the superego, which can be heavily influenced by biological imperatives and what others think of us, respectively. And then, on top of that, we have social norms. These are the things that shape our sexual behaviour. Keep in mind that these things do not develop our sexuality, that primal chemistry was operating at the pleasure center, the only shape how we choose to behave in our social structures.
Bisexual is yet another word in a long series of words invented by humans to be used as a simple label in the attempt to make sense of observed behaviour. The reason there are all these new labels regarding sexuality that we see more different reactions than the previous hetero, homo, and bi. In a sexually liberated society, it is possible to separate behaviour that is purely for pleasure from action that is related to the functioning of a family unit.
This is why a lot of “experimentation” happens at college. It represents a sexually liberated microcosm all of its own — a place where the superego can take a break. And college is not the only microcosm where such behaviour can be observed: There are also camps, military, especially in deployment, prisons, and marooned groups.
So are we all bi? I think the answer is No. We only experience a sexual chemical reaction relating to either sex. There may be countless people who will be attractive to people of the same sex. And they may experience these feelings regularly, or they may only experience them occasionally.
Of course, nobody really knows the answer to this question or any questions similar to this one. Science is still out on whether or not people are inherently born the sexuality that they are, so all we can do is speculate.
Then what is your idea? You can choose to tell us something about your understanding.