Thomas Aquinas,
Summa Theologiae — Question 80 - OF THE USE OR RECEIVING OF THIS SACRAMENT IN GENERAL…
“ But in a measure the sin of fornication makes one more unfit for receiving this sacrament, because thereby especially the spirit becomes enslaved by the flesh, which is a hindrance to the fervor of love required for this sacrament.However, the hindrance to charity itself weighs more than the hindrance to its fervor. Hence the sin of unbelief, which fundamentally severs a man from the unity of the Church, simply speaking, makes him to be utterly unfit for receiving this sacrament ”
