In my experience a lot of both straight people and gay people are both see sexuality as a team sport where someone has to pick a side and treat bi people terribly.

Both see bi people gay but not admitting who they 'really are'. Both see it as the person being indecisive, and also tend to worry about someone being unsatisfied and wanting to always be in a relationship with both sexes at the same time all the time, which is a funny contrast to not picking a side. I'm using sex instead of gender here because people making those comments are always describing the desire as the bi person feeling a desire for the other genitals. I find this pretty ridiculous since those same people that think bi people can't be satisfied while monogamous do think straight and gay people can be.

That isn't everyone of course, but are the main views I have come across in person while supporting friends who came out. That was also the time period where I knew I was straight because of actually spending time thinking about it, and wish more people put more thought into it to help understand that even within the broad categories there are is a ton of variety.