I think it’s in reference to how bi people are often told they are being “greedy” for wanting all/both sexes rather than just having the '“one” the speaker expects. This usually then leads to the misguided conclusion that they are promiscuous and unfaithful - https://lgbtqiacounseling.com/bisexual/bisexual-stereotypes/#Bisexuals_are_Greedy_or_Unable_to_Commit