Disclaimer: I'm a straight cis man, but I have/have had quite a few lesbian friends over time who have explained this joke to me. Take my explanation with the appropriate grain of salt

"the dating pool is tiny to nonexistant" is a common complaint I've heard from lesbians I know, probably related to the fact that I've mostly lived in rural areas. Given the low availability of partners, its more common to jump into relationships quickly and really commit to them. I've also heard a lot of "I really want somebody to live with me so I don't just choke on something and die". Probably less common nowadays, but I'm from a generation when it was harder to be gay publicly --> harder for lesbians to find roommates. Living with other women often meant being stigmatized by straight women that don't understand lesbians are people and not just sex crazed harlots, and honestly that but worse because of a higher threat of violence with straight men. I've heard that moving in with a romantic partner as a lesbian is a move that lets you have a roommate with less objectification related to your gender/orientation. Again, idk how much of that is true today, but that is my understanding of the origins of the joke