First of all, despite bisexuality sometimes being used as an "excuse" for completely homosexual ppl who are not comfortable with being completely open about their sexuality or are simply unsure, no-one will know better about their sexuality than yourself. Even if you suspect someone to "actually" be homosexual/heterosexual, you don't go around telling that person, because if you suspect it, then they probably do too, and are taking their time dealing with it.
Finally, bisexaulity is a real thing. In fact, there is evidence to suggest that we are all on a "bisexual scale", with most ppl leaning more towards attraction to a single gender on on end of the scale. But as very few ppl are "all the way" to one end, then we are all at least a little bit bisexual. I'm guessing the biological explanation would be that it helps us identify if others of the same gender are more or less attractive than us, and measure how much of a threat they pose to taking attention away from possible mates.
On a side note: Personally, I feel that the world would be a better place if we were all fully bisexual.
We would have more ppl to pick from as romantic interests, the world might be less exploitative of a single gender (Not exactly gender equality, but it's something), and since overpopulation is actually a problem, I don't think a lot of ppl should complain that there's a 50% a person may never have children of their own (I say "should" because I know ppl are currently complaining).
