First, because there are "two rabbits walking beside the ground, can Ann tell if I am a male or a female?" This is why comrades are also called rabbits.
Second, because there is no such thing as "homosexuality" in China (let alone "BL" ...), the book takes "rabbit" as the agent. At that time, the feudal society thought did not allow same-sex XX (which would be considered as insulting family status), so it was also a hidden function: male rabbits could mate with male rabbits or female rabbits, but mating with male rabbits would not be fertilized. Every rabbit is like this. To put it bluntly, as long as the male rabbit sees another rabbit or rabbits, whether it is male or female, he will start work. So in ancient times, people didn't know the word homosexuality, so they used rabbits instead.
The so-called "Jade Tao", "Broken Sleeves", "Anling" and "Longyang" all belong to the words of literati. As for the ancient folks, there are many vulgar terms for homosexuality, such as "xianggong", "rabbit", "male prostitute", "little singer", "Xiang brother" and "Qi brother".