Exclusive breastfeeding means the mother's presence is essential, she has to be available most of the time as the primary care giver. Due to the way modern societies and nuclear families work, it's reasonable to assume that it's unlikely both partners would take long term leave to be with the infant, or that the father could be there anywhere near the same amount of time as the mother. So the infant would have far more time to bond with the mother than with the father. Whether this time alone is enough to guarantee a stronger bond is probably worth a separate discussion.