I've already given my Mr. Angry posts about how much I disagree with this proposed rule.
Given that it already happens in Europe (not a rule I don't think, just a "practice" - not sure), and given that some people race in club boats, I assume that once a club boat has a rhombus on it sail it can only be sailed competitively by women. Being club boats they don't tend to have loads of sails for each sex using each boat (funds a bit limited) so I assume that boats end-up being split some dedicated to women and others dedicated to men.
The few clubs I've visited tend to have quite a few boats for general use/hire and one, maybe two to a decent racing spec - and that one (maybe two) is taken off by a club member to competitions (people without their own boat effectively hire the club boat for the day/weekend with trailer, etc.). Thus, first woman who uses it (and puts her rhombus on the sail) then ensures that blokes can no longer use the boat in competitions !!
(In France a lot of people, men and women sail Radials - Standards being the smaller fleets).
I am not illustrating a problem as I don't know what is done about this, just a potential problem to a rule/practice that serves no purpose and is "ill-conceived".
Of course the eventuality will be sailors finding the superior performance inducing optimum number of added panels of sailcloth versus general detraction in performance caused by the added weight aloft
If somebody makes me put a red rhombus on my sail for a specific regatta I will without fussing too much, but I don't understand the reasoning for it to be required in anything other than an open Radial regatta that is also an Olympic qualifying event for the women. It's not like anybody has any trouble telling the guys from the girls. (Although sometimes people think I'm Dave Hartman because our sail numbers are so close)