It's essentially saying that bisexuals cannot truly commit. Again, you're claiming you can read minds and forcing a motive upon others.
I think you don’t understand what being bisexual is or what sexual orientation on a whole is. At the very least you seem to be confusing it and the concept of monogamy.
One doesn’t stop being an orientation simply because they’re in a committed relationship with one gender or another. I don’t stop being heterosexual because I’m married... I’m simply just a heterosexual that is married. I haven’t renounced my heterosexuality, I have declared that my husband is my person, the person I choose above all others and the one I am committed to. I don’t look at other men, I don’t find other men attractive, and I don’t explore situations that could lead to developing attractions on any level, but I’m still heterosexual. I’m capable of the attraction to men, but not seeking it.
If one is bisexual, they enjoy attractions to various people and gender identities. If they choose a person to commit to, then they are stating that they choose that person as a partner, not that they renounce their sexuality. They will remain capable of experiencing attractions to others, however, that doesn’t mean they are and it certainly doesn’t mean they act on them or seek them once they are in a committed relationship.
To be honest, I am heterosexual but if you had asked me in my dating years if I was interested in monogamy, I would have said no. I rather enjoyed being single and exploring all the various forms of attractions one can experience. I didn’t see myself “settling down” until I met my first husband... But even then, it was more of a “I guess this is what people do so it’s what I should do too.” The idea of contentment in monogamy and not experiencing sexual or emotional or intellectual attraction outside of one person for the rest of forever wasn’t something I experienced or thought possible until I met my husband. If you had told me back in the day that I wouldn’t notice or look for or even care about other men, I would have never believed you. How one commits to another and their views on monogamy has nothing to do with their sexuality.