In his January 2017 debate against Tony Costa on the Immaculate Conception, Robert Sungenis made a theological argument in favour of the dogma that, to put it succinctly, if Mary was not conceived without the stain of original sin, Christ would have inherited a fallen, sinful human nature from Mary, and therefore, there would be no perfect atonement.
While I do not believe that Jesus had an immaculate nature, and could have sinned (but he did not [one can find various posts on this blog where I have discussed this and other Christological issues for those interested]), it does appear that many Catholic theologians and apologists would take exception with Sungenis’ argument on two grounds: firstly, Catholic theology holds that the Immaculate Conception was contingent not necessary. Secondly, in Catholic theology, as Jesus was not a Son of Adam as he did not have a biological father, he could not have inherited original sin. As Robert Bellarmine, the patron saint of Sungenis’ own apostolate, wrote the following, which undercuts Sungenis’ misguided apologetic for the Immaculate Conception:
The first foundation is of Christ, Whom the Father hath sanctified and sent into the world [Jn. 10:36]. He did not have original sin, and He could not have it, because He is not a son of Adam with respect to the father; and sin is derived from the father, not the mother. For, if Eve alone had sinned, we would not have original sin. Hence, Christ did not need a redeemer. Besides, the soul of Christ was united to the Word at the very instant of its creation and thus was sanctified through uncreated grace, in a manner in which no pure creature was sanctified. Hence even sanctifying grace was natural to Him, indeed, flowing from the divine nature which was in Him; and in this the sanctification of Christ is preeminent. (The Marian Writings of St. Robert Bellarmine [comp. Casimir Valla; 2011], 54)
It does appear that, even from the perspective of Catholic theology, this approach by Sungenis is without merit.