I really don't agree with you - if attraction was simply and only instinctual, how could you explain that every culture has different standards for what is considered 'beautiful' or 'attractive'? It's more nurture than nature. While the instinct of sex or attraction to the opposite gender is completely instinctual, what you find beautiful is not.
Take certain tribes in Africa, who do not judge by facial beauty, but rather by the jewelery and body modifications that individual has. Or for example, what is considered attractive in say, Korea (pale, slender, dark hair, small lips, large eyes, small faces, slim, V-pointed jaws) is completely different from what is considered attractive in America. Take for example, many Americans find Lucy Liu attractive, because she fits into the American standard of beauty, but most Asians find her only average, or even unattractive.
What you find attractive is heavily influenced by culture, as well as your individual upbringing, interests, and environment. It's not as simple as just instinct. What is considered 'beautiful' or 'attractive' is not universal. If it were solely instinctual, or even mostly instinctual then it probably would be - but there is such a great degree of difference in standards of beauty that I really think it can't be said that it is.
Also, consider that large and muscular is not what is considered successful in all environments. In the hottest parts of Africa, or other places where it is hot, tall and thin was selected for - why? Because if you are tall and thin, with long limbs, it accelerates heat displacement. Shorter and wider is selected for in cold populations. (The Inuit, for example.) Body types evolved in response to environments. (Even today, you can see it. If two short parents from Canada come and raise their child in Texas, the child will usually end up being taller and slimmer than their parents. Where your raise your child has a big effect on growth - children who grow up in sunny places grow faster. It has to do with the synthesis of Vitamin D.) So your perception that it's instinctual to go for the strong and muscular is not true in all cultures. :]