A very possible reason why muscular people (especially men) are more sexually attractive to us is the need to be protected. A woman's brain was set to find a man who could protect her from the dangers of the Stone Age. Therefore, she finds the muscular man more attractive. Another part is the inhibited need to reproduce. If the man looks healthy his genetics are probably so too.
Now what i wonder about is this: In the romanticism, women were most likely attracted to the pale, fat people because they knew why they where fat and pale; they were rich and did not need to have muscle or tan. This makes me wonder. In the romanticism people probably had a great need to be governed financially, which was something they could by being with a wealthy man. In the Stone Age it was different; they needed protection.
Now, in the Modern Age, people are most likely sexually attracted to a muscular man (not saying that anything else is impossible) rather than a man expressing extreme wealth.
All this is sexually speaking, not on the personal level. I want your opinion on what determines a (wo)man's sexual attractiveness nowadays. I hope I wasn't too unclear.