To me - bisexuality makes the most sense, but that's probably because I am, if I wasn't then the sexuality I was would probably make the most sense. To me, though, men can be beautiful, so can women. Men can be nice, so can women. You could love/fancy either (well, I could anyway). I've heard that everyone is bi, or no one is 100% 'one way', but I'm not sure about that. I do know how fluid sexuality can be though, even being bisexual you can sometimes feel 'more straight' or 'more gay' and it varies.
My boyfriend is also bisexual, so I can comment on this firsthand I guess. Although - everyone is different. Some bi people are not monogamous, but some are. As with any sexuality. I don't think because you like both you are more or less likely to be monogamous. The thing about me, and my boyfriend, is we just want each other really. However, we have urges and stuff, and we've both talked about maybe kissing someone of the same sex, or more. We haven't found anyone, so haven't done it, but if we're both OK with it... then it's OK I guess. It's complicated because we can get jealous, but at the same time, of course we will think about the same sex too sometimes. I think if one of us or both of us wanted to not do this, we basically wouldn't, but I think we're quite open minded, so if the situation came along, there's at least a maybe. As long as we were still commited and wouldn't run off with someone else, you know. Having said what I just said, of course bi people can just be with one person if they are in love. Some people might want more, at least on some level, but say the same for some straight/gay people too.
Also as I've said, sexuality is fluid. Even bisexuality can vary from person to person, or vary to one person. For example; I do sometimes think about girls quite a bit, but at other times I mostly just think about guys. I could fancy/love either, but haven't had as much experience with girls. With my boyfriend, he says the same, but says he is generally only attracted to men 'physically' and doesn't often have relationships with them. Although I think he has had some, but he says it's usually more towards women in the physical and emotional sense.