Breasts show to our mind, subconsciously that she is fertile and can give birth. Although there Are special cases where humans are are sterile cant reproduce. As everyone has said before, it's instinct. 40000 years of our body an minds evolving to like the opposite gender simply because its nature. As for homosexuals, I can't tell you. Usually homosexuals are homo is because they had a failing parent in their life so they try to replace that reality with another human being of the same gender. I have no idea about sexual attraction for homosexuals, I don't necessarily want to learn either. I hope this helped a bit.