If two adult males or two adult females hold hands with each other, would it be a sin of rash judgment to automatically assume they are a gay couple?
It is a sin if you assume, based on holding hands, that the couple have done something immoral. Holding hands is a sign of friendship, affection, and perhaps even support or consolation.
Around the world, customs vary. Middle Eastern men hold hands as a sign of friendship. Women in Western cultures often hold hands. American men don’t hold hands much.
I once held hands with a perfect stranger on a plane. She was terrified of flying, and both I and the passenger on her other side held her hands for comfort. Back home, I mentioned it to my wife, and she didn’t mind.
I don’t think in America it would be rash judgment. The holding of hands is a sign of deep affection. Whether that affection extends to anything beyond affection, such as a sexual relationship, is unknown; though any same-sex couple holding hands in public should not be surprised if people think they are homosexual.
Years ago I had a visitor from the East. As we walked toward our building two women were walking ahead of us with their arms around each others waist. My visitor commented that he supposed one sees all kinds of things in Los Angeles.
When we reached check-in one of the women introduced the other to the guard as her daughter whom she hadn’t seen in several years.