As a fella in the age vacinity of late teens/early twenties (21 to be exact), I am probably as good as anyone to answer your question. I tend to find that most decent fellas (I include myself here) never actually go for the best looking girl. I'm not saying that you are not the best looking girl or that Psoriasis makes people worse looking than they are, but some people on these boards are saying how they dont wear skirts or sleeveless/low-cut tops. To be honest, they are not necessary!
Usually the decent fellas do go for personality. This sounds like an old cliche, but it really is true. Obviously there are guys out there who go for anything that moves.(damn, I've lost track of where I was going with this!)
Anyways, so what if by some small chance some fella turns around to you after seeing your Psoriasis and calls you grosse or unclean? What harm will it do? You get depressed for an hour or so until you realise that he was an idiot anyway (obviously) and that you didn't want to be with him anyway!
Think of your Psoriasis as a decent person screening process!