Your statement is a gross oversimplification of the truth.
In general, people prefer healthy (not necessarily thin) partners. As an example, I am not too enamoured with many oriental women because they are just too thin for me. I prefer curvy and BBW women, who are not "thin" but look very healthy, in my opinion.
I am not into obese women because obesity is generally unhealthy. However, a woman who has curves and the "right proportions" are preferred because they appear the healthiest. Thin women do not always appear healthy and therefore are not the most desired.
With respect to women's preferences, they would not necessarily prefer a thin man, especially one without some muscle tone. It is overall health that is prefered and not necessarily weight.....