This makes more sense. The cultural idea that boys should be grossed out by girls, love, and kissing is something that is prevalent in alot of kids media both here and abroad. Having shounen heroes avoid boobs and romance is more likely the response towards the idea that love and affection is gross and gets in the way of the pursuit of adventure.
It's not until a manga is nearly over when love is even a part of the equation, and by then, the young readers are growing old enough to ask themselves why the hero never gave his romantic interest a boob touch after all this time.