Bingo. It's the need for for one of the partners to look and play the part of a man. Maybe it makes them feel less weird to approximate the look of a normal couple.
I have by dint of where I practice a large population of gay and lesbian patients.
Most are fine people. But there is the well-deserved stereotype of the muscular, manly woman who happens to have a girlfriend who is a shrinking violet feminine female.
One of my "regulars" is a truck driver who comes to see me.
If I DIDN'T know she was a woman, I'd honestly think she was a man.
She even has five o'clock stubble
She could easily kick my ass in a fight.
What makes her unique is that she has tattoos all over her body of naked women, spread eagle and "alluring".
If she was a man , I'd think he was some sex-crazed hard-core male.
But she's a woman who REALLY likes other women.