The bi erasure is that the caption is that they’re in a “lesbian relationship” which is erasing the bi person. The second blow against bi people is that the bi person is the cheater, which yes, anyone can be a cheater. It’s just that bi people are often stereotyped as being cheaters because they have more people to cheat with. I do understand this is a silly little comic, this is more about stereotypes and thoughtlessness than about maliciousness on Zach weinersmiths part.