Is bisexuality the default human sexual orientation?

Are we right to categorize people by sexual orientation ? I prefer to think it's the sexual "situation" that turns me on which can vary in type and even gender. I was at an all boys' boarding school and naturally a lot of sexual activity was indulged in between boys of all and differing ages. In the holidays however, we still had sex with girls, not necessarily more enjoyable than the dorm activities at school. It could be less or more according to the situation and one's partner. We didn't think of ourselves as either homosexual or heterosexual - just "sexual". When the new term started, talking and boasting (with some exaggerations, no doubt) about the girls we'd had in the holiday even spiced up our "gay" activities. No "or..or". Just "and".