Bisexual erasure becomes especially harmful when a bisexual person has internalized it. I'm going to get personal for a second and talk about the hardest breakup I've ever been through. After dating for several months, I was falling head over heels in love with a gorgeous young woman. Our male partner was as well. The three of us were great together! Then I went on a trip with my parents and started feeling all kinds of guilt that I wasn't fully out to them about my sexuality and polyamorous lifestyle. (More on my coming out experience later.) I was so happy with our girlfriend and sad that I couldn't share that happiness with my folks. Unfortunately, those feelings made me pull away from her a bit when I got home. I was also scared of being the first person in our triad to say "I love you" because I didn't want to scare her away.
Here comes the erasure part. We broke up. It was terrible. Then later we got to talking and it turned out she was in love too, but thought I was only committed to our relationship for my male partner's sake. Despite the fact that both she and I and the three of us had been having mind-blowing sex for six months, she thought I was just in it for him. This is bisexual erasure at its worst in a polyamorous relationship: the nagging idea that someone might be just a straight girl in it to please her man. This is something straight guys perpetuate every time they approach bisexual and lesbian women, hoping for a threesome. This is also perpetuated by anyone who tells a person "Oh hun, you're not bi, it's just a phase you're going through." (Yes, I personally heard this growing up from both queer and straight friends.) This is why I fight so hard for bisexual visibility and polyamory acceptance - so that we as a community can affect social change and erase these harmful internalized attitudes.