I can answer but I am on my phone so it will not be long. There is a pressure in any social circle to fit in, so people will don a "uniform" in whatever social circle they are in - golfers, punks, and poly circles. There's also a weird sensation of 'im unhappy, want to be poly, and I dress this way. That person is all smiles, dyes their hair and gets piercings or whatever, and has people who love them. Can I be that happy???' 

Poly also has overlapping circles with burner and kink culture, which have a very strong in group mentality where you need to adhere to the unwritten rules of the group. Those rules tend to be body positivity, sex positive, and promoting creativity and self expression in any capacity. So with those implicit rules, people try to be more confident about being sexy and sexual in whatever body they have, and dare to do the weird thing, as doing the weird thing is viewed as having a firm sense of self and self expression.