It's stereotypical. Because media portrays those who are bisexual as tending to have more than one partner, and because so many of us do tend to have more than one partner or want to be in open relationships, it is a common assumption that we will cheat or can't be trusted.
I've always been in open relationships and dated more than one person at a time, several at a time at some points, but they were all aware of one another, and actually were all friends, talked regularly, we got together and socialized, so nothing was kept secret, but as of late, my current boyfriend who is a jealous person and I decided to keep it just he and I, as I was not with anyone else when we began dating, and I can assure anyone who may think otherwise, there is NO chance of me cheating on him
I am 100% against cheating, and I enjoy spending time with him, and am happy having just him in my life. He doesn't have any fears that because of my sexuality and the fact that I've been in open relationships that I will cheat on him, and it is unfortunate that so many people have that misguided impression!