It's not that hard to understand, really. I am bisexual because I would be open to dating and having sex with both women and men. It has nothing to do with threesomes, and I have no desire whatsoever to date more than one person at a time.
It isn't even a conscious choice. I can be attracted to other women. I can fall in love with them, I can lust after them, et cetera. But I can also fall in love with men, lust after men, etc. I never "decided" one day that I wanted to be attracted to women or to men, in both cases it just happened to be the case.