Many become lesbians because of adolescent/pre-adolescent sexual abuse from an older man, which causes them to seek out companionship with women. Another result is that these women decide that being fat is a good way to keep predatory men from them, or make it easier for them to defend themselves by reducing the mass differential between them and the predatory man.
Any obese lesbian that I have every met has either had to deal with child sexual abuse or marital abuse. The unattractive weight - as you say - keeps men at a distance. Even Rosie O’Donnell admits to some kind of sexual abuse as a child.
Then there are the lipstick lesbians - young, attractive women (not unlike the flirtatious Greta Garbo and Louise Brooks and Marlene Dietrich) who like to gender bend. These kind of women seem to drive both men and women crazy.
IMO many of them become lesbian due to positive portrayals of the homosexual lifestyle courtesy of gay activists-indoctrination beginning early in school....