I've heard and read that when guys mate with fertile physically attractive women, there is greater chance of producing healthy offspring and that that's why Guys are attracted to women with wide hips etc.

Ok, but what about the guy? What if it's the skinny nerd trying to mate with a hot girl? What is the benefit to her? He's not going to pass good genes to the offspring if he's skinny and bony and not fit.

The skinny nerd is probably very smart and has great small-muscle control, so will provide genes for survival.

Having healthy offspring depends on far more than two hot people mating.