I disagree. If you committed 10 homicides in a very brief period of time, aim and shoot, aim and shoot, etc, each second, does that mean that you are guaranteed to not be fully culpable? No! Depending on the individual, and judged by God (please read St. Alphonsus and St. Thomas), it is entirely possible to commit a mortal sin in a moment.
Second, it does not follow that just because we “can” commit a mortal sin in a second implies everyone does commit it in a second. But it is also entirely possible that lots of mortal sins of complacencies are committed in a moment. I just wanted to say that it is entirely possible, but definitely not guaranteed. Nonetheless, since hell, an eternity of torture, is at stake, in St. Alphonsus’s words, what folly is it to leave your salvation to a perhaps?
Finally, yes, St. Augustine, St. John Chrysostom, and St. Alphonsus, didn’t merely think the majority of mankind went to Hell, but they thought the vast majority went to Hell.
As the Magisterium declared on February 11, 1661, in Denzinger 2013, on the Material Graveness of Sexual Matters: “Since lightness of matter does not exist in sexual matters, or, if it does, it does not exist in the present case…,” combined with Cardinal Muller saying on EWTN that any attack against the sixth command is a mortal sin, combined with Jesus my Lord and Savior comparing merely looking at a woman lustfully to adultery, combined with the Holy See’s decrees on proximate occasion of sin also being mortal (complacencies), etc, all strongly indicate that complacencies are indeed mortal sins.
As far as “other categories of sin,” these can be mortal too, but it depends on the circumstances and object. But as I stated above, sexual sins, with deliberate consent and knowledge are almost surely mortal! Since nature inclines us to impurity, this sin is definitely the worst of the worst for men between the ages of 15-40. Nature only occasionally tempts us to commit sins of selfishness. However, nature strongly tempts us to commit impurity. Therefore, nowhere near the vigilance that is necessary to overcome sins of impurity is necessary to overcome sins against charity.