/summa/SS/SSIII/SS_Q153:
We must next consider the vice of lust which is opposed to chastity: (1) Lust in general; (2) its species. Under the first head there are five points of inquiry:
(1) What is the matter of lust?
(2) Whether all copulation is unlawful?
(3) Whether lust is a mortal sin?
(4) Whether lust is a capital vice?
(5) Concerning its daughters.