Not all women want big breasts and not all men find big breasts attractive.
However, the reason this is perceived to be commonly desirable is because (1) they are indicators of age, because larger ones can sag more than smaller ones with age, (2) the larger they are, the more babies she could potentially nurse.
The fact that some women could have implants doesn't negate these reasons, because the "why" of how we find them attractive is not analytical cognition (we do not deduce the attractiveness of someone the way you would solve a math problem), but instinctual.
Because our analytical cognition has relatively recently evolved, what our intuition tells us and what our analytical thinking tells us can conflict. These are side effects, so to speak. So then the fact that we *know* that someone's breasts are fake or that we are using contraceptives doesn't make those breasts any less appealing or sex any less pleasurable (because implants and contraceptives were invented too recently to be acted against by evolution).