Jesus gave Himself under the forms of bread and wine. You are not justified in withholding the cup from the laity.

The Catholic Answers

The fact that the Catholic Church does so is sufficient proof that she is justified in doing so. However, let us view the theology of the matter. Jesus gave Himself under both kinds, yet He was completely present in either kind. He who receives either kind receives the whole Christ. In any case, Christ being risen dies no more. It is not possible now to separate Christ’s body and blood in actual fact. Wherever Christ is, there He is whole and entire. He is wholly under the appearances of bread and wholly under the appearances of wine. In receiving the Blessed Sacrament under the form of bread the communicant receives the Blood of Christ also. In receiving under the form of wine alone he would receive the Body also. There is no possibility of receiving the Body of Christ without the Blood of Christ

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty