1. Either one or both girls is "quirky," "alternative," or a "tomboy." Sometimes one of them is an oblivious idiot who just sort of...stumbles into the realization that they like vaginas. In this scenario it's usually best if the other lady is very butch with possibly an arm band tattoo of some form.
2. There is a scene where either one of the main characters has some form of "confrontation" with people in their life because they don't understand why the main character doesn't have a boyfriend because even though she is "quirky," "alternative," or a "tomboy" she is still like...a totally awesome person.
3. The main characters meet! Obligatory sex scene which for some reason always involves nipple licking and lots of super slow kissing, because that's how lesbians have sex!
4. The very next day they are moving in together or at least spending every single waking moment together. (There might at some point be a fight in the relationship, but it's okay because twu wuv always wins out in the end.)
In any case, the main point of the majority of lesbian movies seems to be - it doesn't matter if you actually have anything in common with another person as long as they have genitals that interest you and you can make smoldering temptress looks at each other across a room.
That's enough to base an exclusive, seemingly until the end of time relationship on. Which is not to say that this trope does not exist in hetero cinema or real life relationships. They're just as guilty to, "oh, look, two people met and touched genitals so now they're going to be in love forever!" It's just whenever I encounter these types of relationships in real life that is sort of irritating. You mean you didn't take any time to figure out if the two of you were compatible beyond the way your genitals slot together? And it doesn't matter now because you're "dating" or something?