The lowest common denominator is the least attractive girl among a group of girls; or a single girl that though not attractive is the ONLY girl so if you can fuck her you will.
At a party
John Doe: There's no girls left...
Ben Doe: There's that girl
John Doe: Hell no! She's the lowest common denominator. I do have some self respect!
Ben Doe: Fine then I'll take her...Anyone have a bag?