I was actually having a version of this conversation with my dad this weekend. I was having a hard time explaining how it's optional for me. (well, it used to be, I dunno, I maybe can't go back. I know I can't go back to just one of these two, but if one of them chooses otherwise, I'm not sure what I'd do)
Bisexual doesn't define whether I choose men or women for relationship. It just means that it's possible for me to be with either one. Recognizing that I'm capable of more than one love doesn't mean I'm always going to seek it. I love who I love, and that I'm fortunate enough to have two people that are okay with me loving them both is AWESOME. But should they be taken away by aliens tomorrow (something I think they both might love), I wouldn't stop loving. I would still want people to share my life. I can't say for certain I would or wouldn't seek multiple; but I can't say for certain I could stay in a dyad only. To me, this means OPTIONAL.
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who now lives in a house far away-with stairs I can't climb)