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

We constantly get accused of taking two lovers, thus lowering the amount available for others, despite the incredible challenge of just trying to keep one around when they think you want to cheat on them with someone of the opposite sex.

Leave enough for the rest of us!!