Yes, if wearing a red shirt signals that you are more sexually receptive, women will stop wearing red shirts on dates. If this online trend catches on offline, people will no longer wear red shirts on dates so that they will not be labeled easy. There is a good chance that this will just stay online though.
If the color red is more appealing on dating sites, then that is just a temporary visual cue. Dressing for dates should be more about making a good lasting impression as well as finding common interests with your date. The focus of dating should not be on appearance only since there are many other important values to consider between two people.
I guess if you are looking for sex, it would make sense to wear red in your online dating profile. I, like most people I think, use online dating services to find someone is like-minded. If anything, this makes me want to completely stay away from red, as I won't know what the person is really looking for.
It has been found that wearing red makes one appear more sexually available and this leads to more dates. Isn't this what society is trying to get away from? Aren't we as a people suppose to look for personality above appearance? I feel that I hopefully look more at personality and inner traits than what I am wearing to get myself a date therefore I will not purposefully wear more red when I am trying to get attention.