But consider the case of the bisexual person - contrary to popular belief, most bisexuals don't just feel like they could go either way any time. We also don't get 'the best of both worlds' or a bigger 'choice' of people to date. In fact, we get a smaller dating pool than anyone else, since gay people are often suspicious of us and straight people tend to find anything like that kinda 'icky', and say they wouldn't date a bisexual.
In reality, I can go for long periods of time when I'm only interested in the same sex, and even start to wonder if I'm gay - then suddenly without warning, and it's beyond my own control, I just go right off men and am only interested in women, and can't even imagine myself going with a man and wonder how I ever did. Then it swings round again.
For a bisexual person to commit to one relationship with one person, they're basically being asked to deny an entire part of themselves for the rest of their lives. If I were with a woman, there would be periods where I'd find the concept of my own relationship repulsive, and in order for it to stay alive the woman would have to understand that until my orientation 'swings' back, and neither she nor I have any control over when/whether it does, my heart really wouldn't be in any physical intimacy with her.
In a relationship I was in once, it was extremely difficult for me to keep slogging at a relationship with someone of a gender I wasn't even sure if I'd ever be attracted to it again. It's not that there was any one particular person that I felt tempted by or attracted to - it was more a case of just feeling like I wanted a different relationship, I had needs that were not being met, and couldn't possibly be met, in the relationship I was currently in. I didn't cheat, as I say, but I wouldn't be able to swear that if, at that time, someone of the gender I was oriented towards came along who I felt a strong attraction to, I wouldn't have wavered or even cave.